Fix issues on windows and restore changes from 0.16.1

[nat-n]

Fix various issues manifesting on windows

• Make the tests that break on python37 on windows only run for python >= 3.8
• Reenable disabled test matrix item for windows-python37
• In shell tasks:
  • check for sh installed with git
  • trust git bash over bash on the path which might be a decoy put there to trick us
  • add test for the default powershell, as distinct from pwsh

Restore changes from 0.16.1 that were reverted in 0.16.2

• fixes Poe tries to activate the environment even if poetry config virtualenvs.create false #65 again
• PoetryExecutor does not use poetry if POETRY_VIRTUALENVS_CREATE=false
• Also:
  • PoetryExecutor uses VirtualEnv based execution whenever possible
  • Fixed issue where poe would use the env from the current project instead of
    the target project
  • Update some tests to not avoid encountering missing poetry env

Always explicitly use the poetry virtualenv if possible in the poetry executor

• This removes an optimization that probably does add much value anymore,
  but does seem to cause tasks to be run without the correct virtual_env in
  some scenarios on windows.
• This should fix Version 0.16.1 causes path issues on Windows in Github CI #88

Also make process of updating the path env var to enable a venv try to avoid
redundant updates in case the virtualenv is already active.