Only build alpha builds when PR labelled as Trigger-PR-Build

.github/workflows/release-alpha.yml

@@ -1,17 +1,19 @@
 name: Alpha release
 
 on:
-
-  # Triggers the workflow on any pull request
   pull_request:
+    types: [ labeled, synchronized, opened, reopened ]
 
-  # Allows you to run this workflow manually from the Actions tab
   workflow_dispatch:
 
 jobs:
   build:
     # Don't run for forks as secrets are unavailable.
-    if: github.event.pull_request.head.repo.full_name == github.repository
+    if: |
+      github.event.pull_request.head.repo.full_name == github.repository &&
+      (github.event_name == 'push' ||
+      (github.event_name == 'pull_request' && contains(github.event.pull_request.labels.*.name, 'Trigger-PR-Build')))
+
     name: Release
     runs-on: macos-12