archiver: warn about MSYS2 path translation, fixes #9339

[trxvorr]

This PR addresses issue #9339 by adding a runtime warning when Borg detects it is running under an MSYS2/Git Bash environment without the necessary configuration to prevent path translation.

In MSYS2 environments, POSIX-style paths (e.g., /tmp) are automatically translated to Windows paths (e.g., C:/msys64/tmp) by the shell before being passed to the application. This causes unintended absolute Windows paths to be stored in archives, reducing portability.

Validated that the warning triggers when running in an unmodified MSYS2 shell.
Validated that setting the environment variables suppresses the warning.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.35%. Comparing base (a2ea1e9) to head (d7bbcfb).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9348      +/-   ##
==========================================
- Coverage   83.37%   83.35%   -0.02%     
==========================================
  Files          87       87              
  Lines       15439    15444       +5     
  Branches     2309     2310       +1     
==========================================
+ Hits        12872    12874       +2     
- Misses       1818     1819       +1     
- Partials      749      751       +2     

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[ThomasWaldmann]

Please update your local master branch and then rebase this branch onto that master and force push.

That should solve the CI test failures that are unrelated to your work.

[ThomasWaldmann]

btw, if you address feedback by changing the code, maybe give a short answer to the feedback and/or click on "resolve" if you think it is done.

[ThomasWaldmann]

one nitpick, rest looks good.

can you add a is_msystem-only test that it does not put the windows paths into an archive?

[ThomasWaldmann]

LGTM!

[trxvorr]

@ThomasWaldmann are the codecov failures fine?

[ThomasWaldmann]

Yes.