Add conditional on head_repo on Netlify main GHA job

As per comment

.github/workflows/netlify-main.yaml

@@ -12,7 +12,11 @@ jobs:
     runs-on: ubuntu-latest
     permissions:
       deployments: write
-    if: github.event.workflow_run.conclusion == 'success'
+    # Important: the 'branches' filter above will match the 'main' branch on forks,
+    # so we need to check the head repo too in order to not run on PRs from forks
+    # We check the branch name again too just for completeness
+    # (Is there a nicer way to see if a PR is from a fork?)
+    if: github.event.workflow_run.conclusion == 'success' && github.event.workflow_run.head_repository.full_name == 'vector-im/element-call' && github.event.workflow_run.head_branch == 'main'
     steps:
       - name: Create Deployment
         uses: bobheadxi/deployments@v1