ci(github-actions): enable contentful maintainers to manually approve and run gh actions from PRs that originated from forked repos

Author: ethan-ozelius-contentful

.github/workflows/main.yaml

@@ -5,6 +5,12 @@ permissions:
 on:
   push:
     branches: ['**']
+  pull_request:
+    branches: ['**']
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
 
 jobs:
   build: