lock only force-regenerate on the codegen PR

Author: chingor13

.github/workflows/hermetic_library_generation.yaml

@@ -43,5 +43,5 @@ jobs:
         base_ref: ${{ github.base_ref }}
         head_ref: ${{ github.head_ref }}
         token: ${{ secrets.CLOUD_JAVA_BOT_GITHUB_TOKEN }}
-        force_regenerate_all: ${{ github.event.pull_request.head.ref == 'snapshot-generator' }}
+        force_regenerate_all: ${{ github.event.pull_request.head.ref == 'generate-libraries-main' }}
         image_tag: 2.68.1-SNAPSHOT  # {x-version-update:gapic-generator-java:current}