Properly reference matrix vars

Author: oddbookworm

.github/workflows/run-ubuntu-checks.yml

@@ -125,7 +125,7 @@ jobs:
       if: ${{ steps.gen-coverage.conclusion == 'success' && !cancelled() }}
       uses: actions/upload-artifact@v4
       with:
-        name: pygame-wheels-coverage-{ matrix.os }-{ matrix.python }
+        name: pygame-wheels-coverage-${{ matrix.os }}-${{ matrix.python }}
         path: ./out
 
   # Run cppcheck static analysis on src_c changes