Several enhancements to the Makefile

[abadger]

• Allow arbitrary user supplied arguments for pytest, pylint, and flake8. This can be used, for instance, to select specific tests in pytest (PYTEST_ARGS="-k 'perform_ok'"), or to "disable" a single linter: (PYLINT_ARGS='--version').
• Better document how to determine the proper value for ACTOR=.

[github-actions]

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[pirat89]

Nice change! :)