build: fall back to previous image version on release PRs

[meltsufin]

The library generation pipeline fails on release PRs because it attempts to pull a Docker image version that has not yet been built or pushed to the registry.

This PR adds a fallback mechanism in hermetic_library_generation.sh to use the previous version of the image from the main branch if the requested version fails to pull on a release PR.

To verify this in CI, this branch is named with the prefix 'release-please--' to simulate a release PR.

Fixes #12825

[gemini-code-assist]

Code Review

This pull request introduces a fallback mechanism for the hermetic library generation script. When a release PR branch is detected, the script attempts to pull the specified Docker image; if it fails, it tries to extract and use the image tag from the target branch's workflow configuration. The review feedback suggests improving the robustness of the tag extraction logic to better handle potential multiple matches, comments, or colons within the YAML file.

[meltsufin]

Fallback validated in a separate release simulation PR #12849:

+ [[ release-please--simulation =~ ^release-please-- ]]
+ echo 'Detected release PR branch: release-please--simulation'
+ docker pull gcr.io/cloud-devrel-public-resources/java-library-generation:9.99.9
Detected release PR branch: release-please--simulation
Error response from daemon: manifest for gcr.io/cloud-devrel-public-resources/java-library-generation:9.99.9 not found: manifest unknown: Failed to fetch "9.99.9"
+ echo 'Image not found for version 9.99.9. Falling back to previous version from main.'
Image not found for version 9.99.9. Falling back to previous version from main.
++ git show main:.github/workflows/hermetic_library_generation.yaml
++ grep -m 1 '^[[:space:]]*image_tag:'
++ cut -d : -f 2-
++ cut -d '#' -f 1
++ xargs
+ previous_tag=2.71.0
+ '[' -n 2.71.0 ']'
+ echo 'Using previous image version: 2.71.0'
+ image_tag=2.71.0
Using previous image version: 2.71.0
++ id -u
++ id -g
++ pwd
+ docker run --rm -u 1001:1001 -v /home/runner/work/google-cloud-java/google-cloud-java:/workspace -v /tmp/tmp.dG9h7KnPTo:/workspace/googleapis -e GENERATOR_VERSION=2.71.0 gcr.io/cloud-devrel-public-resources/java-library-generation:2.71.0 --generation-config-path=/workspace/generation_config.yaml --library-names= --api-definitions-path=/workspace/googleapis
Unable to find image 'gcr.io/cloud-devrel-public-resources/java-library-generation:2.71.0' locally
2.71.0: Pulling from cloud-devrel-public-resources/java-library-generation

[sonarqubecloud]

Quality Gate passed for 'gapic-generator-java-root'

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[sonarqubecloud]

Quality Gate passed for 'gapic-generator-java-root'

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud