Don't install ruff in the baseline container

ruff does not support python 3.6 anymore (and we should probably drop it
here as well), so only check that the tests still pass with the minimum
python version rather than also running the linters. Any problems they
find should be discovered by the other jobs.

.github/workflows/python-app.yml

@@ -20,12 +20,7 @@ jobs:
                 run: |
                     # Run as a non-root user to avoid cheribuild errors
                     adduser --disabled-password --gecos "Not Root" notroot
-                    su -c "python -m pip install --user --upgrade pip" notroot
-                    su -c "pip install --user -r requirements.txt" notroot
-            -   name: Lint with flake8
-                run: |
-                    # stop the build if there are any flake8 warnings
-                    su -c "python -m flake8" notroot
+                    su -c "python -m pip install --user --upgrade pip pytest" notroot
             -   name: Run basic regression tests
                 run: su -c "tests/run_basic_tests.sh" notroot
     build-macos: