GHA/windows: log preinstalled MSYS2 packages

CI jobs rely on defaults, sometimes to reduce log noise (possibly
improve performance a bit) or simplify, sometimes by accident. List them
to help debug or optimize on changes.

Refs:
https://github.com/actions/runner-images/issues/14562#issuecomment-5301777198
636489312c
https://github.com/msys2/MSYS2-packages/pull/6491
https://github.com/msys2/MSYS2-packages/pull/6479

Follow-up to d854ab4673 #22580
Follow-up to c349bd668c #14097

Closes #22589

.github/workflows/windows.yml

@@ -329,6 +329,10 @@ jobs:
               install: 'mingw-w64-i686-c-ares mingw-w64-i686-gnutls mingw-w64-i686-libssh2 mingw-w64-i686-openssl' }
       fail-fast: false
     steps:
+      - name: 'preinstalled tools'
+        shell: bash
+        run: echo '::group::MSYS2 packages'; /c/msys64/usr/bin/pacman.exe -Q || true; echo '::endgroup::'
+
       - uses: msys2/setup-msys2@66cd2cce69caa17b53920067426061ca1de3a884 # v2.32.0
         if: ${{ matrix.sys == 'msys' }}
         with:
@@ -647,6 +651,10 @@ jobs:
             chkprefill: ''  # Set it once to silence actionlint
       fail-fast: false
     steps:
+      - name: 'preinstalled tools'
+        shell: bash
+        run: echo '::group::MSYS2 packages'; /c/msys64/usr/bin/pacman.exe -Q || true; echo '::endgroup::'
+
       - uses: msys2/setup-msys2@66cd2cce69caa17b53920067426061ca1de3a884 # v2.32.0
         with:
           msystem: ${{ matrix.sys }}
@@ -1006,6 +1014,10 @@ jobs:
 
       fail-fast: false
     steps:
+      - name: 'preinstalled tools'
+        shell: bash
+        run: echo '::group::MSYS2 packages'; /c/msys64/usr/bin/pacman.exe -Q || true; echo '::endgroup::'
+
       - uses: msys2/setup-msys2@66cd2cce69caa17b53920067426061ca1de3a884 # v2.32.0
         with:
           msystem: ${{ matrix.arch == 'arm64' && 'clangarm64' || 'ucrt64' }}