alr overrides the git from PATH with the MSYS2 version

[Seb-MCaw]

Checklist

• I've searched for similar issues and found none.
• I've included the output of alr version.
• I've included complete steps to reproduce my issue.

Describe the bug

alr won't auto-install the MSYS2 version of git itself if the native Windows version is already present, but will use it (in preference to the Windows version, regardless of what's on PATH) if you install it manually with pacman. There are non-trivial differences between the two, so workspaces the user is manipulating with the native git will cause alr errors.

Since the MSYS2 version of git isn't currently supported by Alire (12 failures when running testsuite with MSYS2 git installed as described below, mostly because Alire chokes on Unix paths when running on Windows), in the short term alr should probably never auto-install it, and exclude it from any PATH modifications.

To Reproduce

On Windows, in PowerShell (or presumably any terminal other than MinGW) ...

1. pacman --sync git
2. Ensure that this new git is not on PATH
3. git clone https://github.com/mosteo/aaa
4. cd aaa
5. git status reports a clean repo
6. alr publish --skip-submit fails due to unstaged changes
7. pacman --remove git
8. alr publish --skip-submit now succeeds