[WIP] Optional local MSYS2 setup.

[Sopel97]

Potentially fixes #189.

[Sopel97]

A different solution to this will be made ~tomorrow. See discussion in the linked issue.

[Sopel97]

Okay. So this is the current solution (Windows only, linux not impacted):

Added --msys2-path CLI argument that needs to be specified if the user wants to use an existing msys2 installation. PATH will be updated with .../msys2/, .../msys2/mingw64/bin, .../msys2/usr/bin - but I'm not actually certain that this environment will be propagated all the way through to the required subprocesses... This needs testing.

Otherwise --allow-choco must be set to True (just so we don't install a lot of stuff without user's consent), and the script will attempt to install Chocolatey and later MSYS2 through it. This is a local installation in the workspace directory, without automatically setting the PATH. The PATH will be setup in the local environment the same as in the case of --msys2-path, in which case WE MAY HAVE A PROBLEM if it doesn't work. Needs testing.

Additionally, choco requires TLS 1.2 and powershell 3.0, which are not always present on windows 7 and may require additional setup. So Windows 7 users will likely need to install msys2 manually and use --msys2-path.

@ppigazzini Could you perhaps test this in your windows sandbox? I'm mostly worried that the PATH setup doesn't propagate enough.

[Sopel97]

Do we want to maintain this or should we rely on the user to install MSYS?

[vondele]

I think that should be done by the user.