feat: push SNAPSHOT versions of the hermetic build docker image (#2888)

This PR simplifies the setup of image publication. It will now rely on a
single Cloud Build workflow that will push an image with three tags
whenever there is a push to the `main` branch (i.e. merged PR):
 - An image tagged with the commit SHA of the merged commit
 - An image tagged `latest`
- An image tagged with the generator version (e.g. 1.1.1-SNAPSHOT or
1.1.1 when it's a release PR)

** Follow ups **
 - [ ] disable the release trigger in Cloud Build

.cloudbuild/library_generation/cloudbuild-library-generation-push.yaml

@@ -15,19 +15,23 @@
 timeout: 7200s # 2 hours
 substitutions:
   _IMAGE_NAME: "gcr.io/cloud-devrel-public-resources/java-library-generation"
+  _GAPIC_GENERATOR_JAVA_VERSION: '2.41.1-SNAPSHOT' # {x-version-update:gapic-generator-java:current}
   _SHA_IMAGE_ID: "${_IMAGE_NAME}:${COMMIT_SHA}"
   _LATEST_IMAGE_ID: "${_IMAGE_NAME}:latest"
+  _VERSIONED_IMAGE_ID: "${_IMAGE_NAME}:${_GAPIC_GENERATOR_JAVA_VERSION}"
 steps:
   # Library generation build
   - name: gcr.io/cloud-builders/docker
     args: [
       "build",
       "-t", "${_SHA_IMAGE_ID}",
       "-t", "${_LATEST_IMAGE_ID}",
+      "-t", "${_VERSIONED_IMAGE_ID}",
       "--file", ".cloudbuild/library_generation/library_generation.Dockerfile", "."]
     id: library-generation-build
     waitFor: ["-"]
 
 images:
   - ${_SHA_IMAGE_ID}
   - ${_LATEST_IMAGE_ID}
+  - ${_VERSIONED_IMAGE_ID}