build: force fallback by using non-existent image version

This commit updates the image_tag to a non-existent version (9.99.9) in hermetic_library_generation.yaml to force the pull failure and verify the fallback logic works in CI.

DO NOT MERGE THIS PR

Author: meltsufin

.github/workflows/hermetic_library_generation.yaml

@@ -44,4 +44,4 @@ jobs:
         head_ref: ${{ github.head_ref }}
         token: ${{ secrets.CLOUD_JAVA_BOT_GITHUB_TOKEN }}
         force_regenerate_all: ${{ github.event.pull_request.head.ref == 'generate-libraries-main' }}
-        image_tag: 2.71.0  # {x-version-update:gapic-generator-java:current}
+        image_tag: 9.99.9