Run CMake file auto-generation only on the main protobuf repo (#10611)

This workflow will otherwise fail on forks, so let's make sure to do it
only on the main repo where we need it.

.github/workflows/generated_cmake.yml

@@ -11,6 +11,7 @@ on:
 
 jobs:
   cmake:
+    if: github.repository == 'protocolbuffers/protobuf'
     runs-on: ubuntu-latest
 
     strategy: