Windows CI: use runs-on directly

No need to put the os in the matrix as the job only runs on Windows.

Author: MisterDA

.github/workflows/windows.yml

@@ -13,12 +13,10 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os:
-          - windows-latest
         windows_env:
           - cygwin
           - msys2
-    runs-on: ${{ matrix.os }}
+    runs-on: windows-latest
     steps:
       - name: Checkout tree
         uses: actions/checkout@v6