Add the SKIP envar to bypass specific actions

While testing, it is painful to have mypy running or even the test failure to rerun.
We could add an envar SKIP` that can be a string of actions to bypass:

• flake8 to skip code style checks.
• mypy to skip type annotations checks.
• rerun to not rerun failed test(s).
• integration to notrun integration tests on Windows.
• all to skip all above (equivalent to flake8,mypy,rerun,integration).
• tests to run only code quality checks.

Usage:

SKIP=mypy CMD
SKIP=mypy,flake8 CMD
SKIP=all CMD