Use the host Python to handle the build

[lazka]

This way building packages doesn't depend on msys2 packages

@eine

[eine]

It seems that this PR includes three features:

• Using the host Python instead of packages python and python-pip installed through pacman. I think this is an adequate change.
• Executing Python from Powershell instead of doing it from MSYS2's bash.
  • Calling msys2.cmd instead of bash, from Python.

The last point makes the pytest script non-portable. It will work in GHA only, because msys2.cmd is not expected to exist in any other environment. Hence, I think it would be desirable to call Python from MSYS2's bash, thus preserving the current buildqueue.py.

[lazka]

OK, so your goal is to make the build script work on GHA and on a local machine?

[eine]

@lazka, yes. For two purposes:

• Make it easier to debug problems with the script/logic.
• Allow to distribute the workload. The "local machine" might be another CI service, or the server/VPS of some maintainer. Similarly to other distributed systems, where multiple agents can "compete" to be the first in solving some issue (in this case, building a package). Anyway, ideally, if multiple machines are available, it would make sense to schedule the workload explicitly.

[lazka]

OK, sounds good, I'll keep that in mind.