Only run build action in main repo

.github/workflows/build-push-template.yml

@@ -7,6 +7,7 @@ on:
 
 jobs:
   build-push-template:
+    if: ${{ github.repository == 'substrate-developer-hub/substrate-node-template' }}
     runs-on: ubuntu-18.04
     steps:
     - uses: actions/checkout@v2