GHA/windows: work around Git for Windows perf regression

GHA/windows: work around Git for Windows perf regression

Fix the significant perf regression for vcpkg jobs by switching to the
MSYS2 shell environment from Git for Windows. This env is already used
for old-mingw-w64 job that remained unaffected by this issue.

The issue began with the windows-runner update 20241015.1.0. It bumped
Git for Windows from Git 2.46.2.windows.1 to Git 2.47.0.windows.1. GfW
bumped its MSYS2 components, including `msys-2.0.dll`. That's Cygwin
code, which may have contributed to this. Pipes were involved and
`runtests.pl` relies on pipes heavily in parallel mode. (The issue was
not seen with parallel tests disabled, in retrospect.)

This is useful as a permanent solution too. It drop GfW as a dependency
and makes Windows jobs use one less shell/env flavour.

Long term it might help to use native Windows Perl to avoid the MSYS
layer completely, if there is a way to make that work.

Assortment of possibly related links:
https://cygwin.com/pipermail/cygwin/2024-August/256398.html
https://github.com/cygwin/cygwin/commit/f78009cb1ccf84cc343cf2441c76196461d87532
https://github.com/cygwin/cygwin/commit/7f3c22532577ae0a926e8eb8ad63787c9841abbf

https://github.com/actions/runner-images/issues/10843
https://github.com/git-for-windows/git/issues/5199
https://github.com/git-for-windows/msys2-runtime/pull/75
https://github.com/git-for-windows/msys2-runtime/commit/7913a41703dbc476ad3cf1b85e6939ebbe524251
https://github.com/git-for-windows/msys2-runtime/commit/555afcb2f3a6638084912ce1011bd6acef59ea79
https://github.com/cygwin/cygwin/commit/1c5f4dcdc5ec3344e3fd741c43fa359d0e1323c0

Follow-up to c33174d42fc8a4a0625b46f1d09f5e79eb2abbf1 #15364
Follow-up to 1e0305973c22b1d84036fe0c4eee34aea5cd40cc #15356

Closes #15380