github action: only run in the main repo

This prevents the action being run in the fork, which
will generate errors due to missing token.

.github/workflows/generate_formulae.brew.sh_data.yml

@@ -7,6 +7,7 @@ on:
 
 jobs:
   generate:
+    if: github.repository == 'Homebrew/homebrew-core'
     runs-on: macos-latest
     steps:
       - uses: actions/checkout@master