gh-actions: execute terraform apply only on push to main

.github/workflows/terraform.yml

@@ -42,6 +42,7 @@ jobs:
   terraform-plan:
     runs-on: ubuntu-latest
     needs: terraform-check
+    if: github.event_name == 'push' && github.ref == 'refs/heads/main'
     env:
       TF_VAR_account_id: ${{ secrets.AWS_ACCOUNT_ID }}
       TF_VAR_gh_username: ${{ secrets.TF_VAR_GH_USERNAME }}