Work around a GitHub Actions regression

.github/workflows/ci.yml

@@ -90,6 +90,17 @@ jobs:
       with:
         arch: ${{matrix.arch}}
 
+    - name: Install MSYS2
+      if: matrix.compiler == 'mingw'
+      uses: msys2/setup-msys2@v2
+      with:
+        update: true
+        install: mingw-w64-x86_64-ninja
+
+    - name: Put MSYS2_MinGW64 on PATH
+      if: matrix.compiler == 'mingw'
+      run: echo "D:/a/_temp/msys64/mingw64/bin" | Out-File -FilePath $env:GITHUB_PATH -Encoding utf8 -Append
+
     - name: Install apt dependencies (Linux)
       if: runner.os == 'Linux'
       run: |