github CI: make msys2 shell the default

Reduces verbosity when installing packages.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

.github/workflows/on_PR_meson.yaml

@@ -83,6 +83,9 @@ jobs:
       matrix:
         deps: ['enabled', 'disabled']
         platform: ['UCRT64', 'CLANG32', 'CLANG64']
+    defaults:
+      run:
+        shell: msys2 {0}
     steps:
       - uses: actions/checkout@v3
 
@@ -100,7 +103,6 @@ jobs:
             pkgconf:p
 
       - name: Compile and Test
-        shell: msys2 {0}
         run: |
           meson setup "${{github.workspace}}/build" -Dauto_features=${{matrix.deps}} -Dwarning_level=3 -Dcpp_std=c++20
           meson compile -C "${{github.workspace}}/build" --verbose