ci: Windows: Replace `MINGW_INSTALLS` with `MINGW_ARCH`

Currently the Windows build generates a warning

   => WARNING: MINGW_INSTALLS is deprecated, use MINGW_ARCH instead

Make this change to the CI workflow file.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>

.github/workflows/test.yml

@@ -84,7 +84,7 @@ jobs:
       run:
         shell: msys2 {0}
     env:
-      MINGW_INSTALLS: ${{ matrix.msystem }}
+      MINGW_ARCH: ${{ matrix.msystem }}
     steps:
 
     - run: git config --global core.autocrlf input