Improve environment isolation in Cygwin and Alpine Linux on CI

The main change here is to the Cygwin test workflow, which now runs
the tests in a venv (a Python virtual environment), to avoid mixing
up our intended dependencies with other files installed by Python
related packages on the system. This should either fix the problem
where `test_handle_process_output` reports an extra line in stderr
for the `cat_file.py` subprocess on CI or Cygwin, or at least make
it somewhat easier to continue investigating the problem. I think
this change is also valuable for its own sake.

The connection to the `test_handle_process_output` failure is that
the extra line in stderr appears at the beginning and is an error
message about a missing import needed for subprocess code coverage.
With the recently added more detailed error reporting, it shows:

            self.assertEqual(line_counts[1], expected_line_count, repr(lines[1]))
    >       self.assertEqual(line_counts[2], expected_line_count, repr(lines[2]))
    E       AssertionError: 5003 != 5002 : ['pytest-cov: Failed to setup subprocess coverage. Environ: {\'COV_CORE_SOURCE\': \'git\', \'COV_CORE_CONFIG\': \':\', \'COV_CORE_DATAFILE\': \'/cygdrive/d/a/GitPython/GitPython/.coverage\'} Exception: ModuleNotFoundError("No module named \'iniconfig\'")\n', 'From github.com:jantman/gitpython_issue_301\n', ' = [up to date]      master     -> origin/master\n', ' = [up to date]      testcommit1 -> origin/testcommit1\n', ' = [up to date]      testcommit10 -> origin/testcommit10\n', ' = [up to date]      testcommit100 -> origin/testcommit100\n', ' = [up to date]      testcommit1000 -> origin/testcommit1000\n', ' = [up to date]      testcommit1001 -> origin/testcommit1001\n', ' = [up to date]      testcommit1002 -> origin/testcommit1002\n', ' = [up to date]      testcommit1003 -> origin/testcommit1003\n', ' = [up to date]      testcommit1004 -> origin/testcommit1004\n', ' = [up to date]      testcommit1005 -> origin/testcommit1005\n', ' = [up to date]      testcommit
    test/test_git.py:793: AssertionError

This could possibly be worked around by attempting to install a
package to provide `iniconfig`, by configuring `pytest-cov` in a
way that does not require it, or by temporarily disabling code
coverage reports on Cygwin. Before exploring those or other
options, this change seeks to prepare a more isolated environment
in which different package versions are more likely to work
properly together.

In addition to that change to the Cygwin workflow, this also
changes the way the Alpine Linux test workflow is made to use a
venv, using the technique that is used here, and undoing some
changes in the Alpine Linux workflow that should not be necessary.
The reason for making that change together with this Cygwin change
is that if either does not work as expected, it might shed light on
what is going wrong with the other.

Although the same technique is used on Cygwin and in Alpine Linux,
it looks a little different, because the environment variable on
Cygwin is `BASH_ENV` (since script steps are run in `bash`), while
the environment variable is `ENV` (since script steps are run in
`busybox sh`) and this must also be allowed to pass through `sudo`
(since `sh`, which is `busybox sh` on this system, is being invoked
through `sudo`).

.github/workflows/alpine-test.yml

@@ -16,8 +16,8 @@ jobs:
     steps:
     - name: Prepare Alpine Linux
       run: |
-        apk add sudo git git-daemon python3 py3-pip py3-setuptools py3-virtualenv py3-wheel
-        echo 'Defaults env_keep += "CI GITHUB_* RUNNER_*"' >/etc/sudoers.d/ci_env
+        apk add sudo git git-daemon python3 py3-pip py3-virtualenv
+        echo 'Defaults env_keep += "CI ENV GITHUB_* RUNNER_*"' >/etc/sudoers.d/ci_env
         addgroup -g 127 docker
         adduser -D -u 1001 runner  # TODO: Check if this still works on GHA as intended.
         adduser runner docker
@@ -47,17 +47,19 @@ jobs:
     - name: Set up virtualenv
       run: |
         python -m venv .venv
-        . .venv/bin/activate
-        printf '%s=%s\n' 'PATH' "$PATH" 'VIRTUAL_ENV' "$VIRTUAL_ENV" >>"$GITHUB_ENV"
+        echo 'ENV=.venv/bin/activate' >> "$GITHUB_ENV"  # ENV (not BASH_ENV) for BusyBox sh.
+
+    - name: Update PyPA packages
+      run: |
+        # Get the latest pip, wheel, and prior to Python 3.12, setuptools.
+        python -m pip install -U pip $(pip freeze --all | grep -ow ^setuptools) wheel
 
     - name: Install project and test dependencies
       run: |
-        . .venv/bin/activate
         pip install ".[test]"
 
     - name: Show version and platform information
       run: |
-        . .venv/bin/activate
         uname -a
         command -v git python
         git version
@@ -66,5 +68,4 @@ jobs:
 
     - name: Test with pytest
       run: |
-        . .venv/bin/activate
         pytest --color=yes -p no:sugar --instafail -vv

.github/workflows/cygwin-test.yml

@@ -55,10 +55,10 @@ jobs:
         # and cause subsequent tests to fail
         cat test/fixtures/.gitconfig >> ~/.gitconfig
 
-    - name: Ensure the "pip" command is available
+    - name: Set up virtualenv
       run: |
-        # This is used unless, and before, an updated pip is installed.
-        ln -s pip3 /usr/bin/pip
+        python -m venv .venv
+        echo 'BASH_ENV=.venv/bin/activate' >>"$GITHUB_ENV"
 
     - name: Update PyPA packages
       run: |