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
cygwin/cygwin@f78009c
cygwin/cygwin@7f3c225

actions/runner-images#10843
git-for-windows/git#5199
git-for-windows/msys2-runtime#75
git-for-windows/msys2-runtime@7913a41
git-for-windows/msys2-runtime@555afcb
cygwin/cygwin@1c5f4dc

Follow-up to c33174d curl#15364
Follow-up to 1e03059 curl#15356

Closes curl#15380

Author: vszakats

.github/workflows/windows.yml

@@ -614,7 +614,7 @@ jobs:
     timeout-minutes: 55
     defaults:
       run:
-        shell: bash
+        shell: C:\msys64\usr\bin\bash.exe {0}
     env:
       VCPKG_BINARY_SOURCES: 'clear;x-gha,readwrite'
       VCPKG_DISABLE_METRICS: '1'
@@ -737,6 +737,7 @@ jobs:
       - name: 'cmake configure'
         timeout-minutes: 5
         run: |
+          PATH="/c/msys64/usr/bin:$PATH"
           cmake -B bld ${options} \
             "-DCMAKE_TOOLCHAIN_FILE=$VCPKG_INSTALLATION_ROOT/scripts/buildsystems/vcpkg.cmake" \
             "-DVCPKG_INSTALLED_DIR=$VCPKG_INSTALLATION_ROOT/installed" \
@@ -762,7 +763,9 @@ jobs:
 
       - name: 'cmake build'
         timeout-minutes: 5
-        run: cmake --build bld --config '${{ matrix.type }}' --parallel 5
+        run: |
+          PATH="/c/msys64/usr/bin:$PATH"
+          cmake --build bld --config '${{ matrix.type }}' --parallel 5
 
       - name: 'curl version'
         timeout-minutes: 1
@@ -776,7 +779,9 @@ jobs:
       - name: 'cmake build tests'
         if: ${{ matrix.tflags != 'skipall' }}
         timeout-minutes: 10
-        run: cmake --build bld --config '${{ matrix.type }}' --parallel 5 --target testdeps
+        run: |
+          PATH="/c/msys64/usr/bin:$PATH"
+          cmake --build bld --config '${{ matrix.type }}' --parallel 5 --target testdeps
 
       - name: 'install test prereqs'
         if: ${{ matrix.tflags != 'skipall' && matrix.tflags != 'skiprun' }}
@@ -792,18 +797,20 @@ jobs:
 
       - name: 'cmake run tests'
         if: ${{ matrix.tflags != 'skipall' && matrix.tflags != 'skiprun' }}
-        timeout-minutes: 12
+        timeout-minutes: 10
         run: |
           export TFLAGS='-j8 ~WebSockets ~SCP ~612 ${{ matrix.tflags }}'
-          TFLAGS+=' ~987'  # 'SMTPS with redundant explicit SSL request'
           if [[ '${{ matrix.install }}' = *'libssh2[core,zlib]'* ]]; then
             TFLAGS+=' ~SFTP'
           elif [[ '${{ matrix.install }}' = *'libssh '* ]]; then
             TFLAGS+=' ~614'  # 'SFTP pre-quote chmod' SFTP, pre-quote, directory
           fi
           PATH="$PWD/bld/lib:$PATH:/c/Program Files (x86)/stunnel/bin:/c/Program Files/OpenSSH-Win64"
+          PATH="/c/msys64/usr/bin:$PATH"
           cmake --build bld --config '${{ matrix.type }}' --target test-ci
 
       - name: 'cmake build examples'
         timeout-minutes: 5
-        run: cmake --build bld --config '${{ matrix.type }}' --parallel 5 --target curl-examples
+        run: |
+          PATH="/c/msys64/usr/bin:$PATH"
+          cmake --build bld --config '${{ matrix.type }}' --parallel 5 --target curl-examples