CI: explicitely CC/CXX for clang only mingw environments

MSYS2 has stopped installing gcc compatibility binaries in clang environments
by default some time ago, and distutils is currently hardcoded to look for "gcc",
while only cc/c++ and clang/clang++ are in PATH.

Work around for now by explicitely setting CC/CXX to override the defaults.

Idealy distutils would try to look harder for a valid compiler before giving up,
but this can be improved in the future.

.github/workflows/main.yml

@@ -124,10 +124,10 @@ jobs:
     strategy:
       matrix:
         include:
-          - { sys: mingw64, env: x86_64 }
-          - { sys: mingw32, env: i686 }
-          - { sys: ucrt64,  env: ucrt-x86_64 }
-          - { sys: clang64, env: clang-x86_64 }
+          - { sys: mingw64, env: x86_64, cc: gcc, cxx: g++ }
+          - { sys: mingw32, env: i686, cc: gcc, cxx: g++ }
+          - { sys: ucrt64,  env: ucrt-x86_64, cc: gcc, cxx: g++ }
+          - { sys: clang64, env: clang-x86_64, cc: clang, cxx: clang++}
     runs-on: windows-latest
     steps:
       - uses: actions/checkout@v4
@@ -155,6 +155,9 @@ jobs:
           pip install -e .[test]
       - name: Run tests
         shell: msys2 {0}
+        env:
+          CC: ${{ matrix.cc }}
+          CXX: ${{ matrix.cxx }}
         run: |
           source /tmp/venv/bin/activate
           pytest