build nightly only from main branch

Signed-off-by: Michael Kaufmann <d00p@froxlor.org>

.github/workflows/build-mariadb.yml

@@ -53,7 +53,7 @@ jobs:
     name: Create nightly/testing tarball
     runs-on: ubuntu-latest
     needs: froxlor
-    if: ${{ github.event_name == 'push' }}
+    if: ${{ github.event_name == 'push' }} && ${{ github.ref == 'refs/heads/main' }}
 
     steps:
       - name: Checkout