Run nightly on php org only unless triggered via workflow_dispatch

.github/workflows/nightly.yml

@@ -6,6 +6,7 @@ on:
 jobs:
   GENERATE_MATRIX:
     name: Generate Matrix
+    if: github.repository_owner == 'php' || github.event_name == 'workflow_dispatch'
     runs-on: ubuntu-latest
     outputs:
       branches: ${{ steps.set-matrix.outputs.branches }}