CI: only upload the artifact for one of the compilers

Author: whot

.github/workflows/build.yml

@@ -54,6 +54,7 @@ jobs:
         run: |
           mv _build/xf86-input-wacom-*tar.bz2 .
       - uses: actions/upload-artifact@v4
+        if: ${{ matrix.compiler == 'gcc' }}
         with:
           name: tarball
           path: xf86-input-wacom-*tar.bz2