Do not run GitHub workflows on forks (#1107)

.github/workflows/gh-pages.yml

@@ -10,8 +10,9 @@ on:
 jobs:
   build-and-deploy:
     permissions:
-     contents: write
+      contents: write
     runs-on: ubuntu-latest
+    if: github.repository == 'flutter/samples'
 
     steps:
       - name: Checkout