fix: restore hermetic build image publication

[diegomarquezp]

So far, the Release Please config did not consider .cloudbuild/library_generation/cloudbuild-library-generation-push.yaml to update the tags.

This PR fixes this and also updates the image tag to the current one (2.42.0).

Follow ups

• [ ] Restore the image tag to the current SNAPSHOT

[sonarcloud[bot]]

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 SonarCloud

[sonarcloud[bot]]

Quality Gate passed for 'java_showcase_integration_tests'

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 SonarCloud