Address build failures due to Python 3 binary lookup

[eramongodb]

Partially reverts some changes made in #352 concerning Python 3 binary lookup (motivated by a stray explicit call to python) to address build failures on some EVG distros where neither python nor python3 are available in default binary paths.

The Using Python3 binary: $PYTHON case added in #352 is preserved to avoid a redundant call to find_python3 when start-orchestration.sh is invoked via run-orchestration.sh (which already called find_python3). This might still break EVG builds that happen to set the PYTHON environment variable before invoking start-orchestration.sh directly (instead of through run-orchestration.sh) if PYTHON is set to a Python binary that is not 3.0+ or does not support venv. If that is the case, this too can be reverted to the prior unconditional call to find_python3.

[blink1073]

Test failure: find-python3.sh must be run in a Bash shell!

[blink1073]

LGTM, thanks!

[eramongodb]

Although the GitHub Actions config was updated to use shell: bash for consistency, the run-orchestration.sh script itself still uses #!/bin/sh for backwards-compatibility, as many DET users still run sh run-orchestration.sh. Making run-orchestration.sh require Bash would be a change far beyond the scope of this PR; therefore, a Bash subshell is used to run find_python3 instead. Documented this context with a comment prior to running find_python3.

[blink1073]

Test failure is unrelated, was fixed in #356