GAR Integration: Add predefined CI variables

Now that New Google Artifact Registry Project Integration (#425066 - closed) has been merged we can create new predefined environment variables to facilitate the interaction with GAR in CI:

• GOOGLE_ARTIFACT_REGISTRY_PROJECT_ID
• GOOGLE_ARTIFACT_REGISTRY_REPOSITORY_NAME
• GOOGLE_ARTIFACT_REGISTRY_REPOSITORY_LOCATION

The above should be filled from the corresponding GAR integration settings.

This change should be gated behind the global Google Artifact Registry gcp_artifact_registry feature flag.

Plan

1. New Google Artifact Registry Project Integration (#425066 - closed)
2. GAR Integration: Custom client class (#425147 - closed)
3. GraphQL: get GAR artifacts from project (#425149 - closed)
4. GraphQL: get GAR artifact details query (#425150 - closed)
5. Add predefined CI variables. (👈 this issue)
6. GAR integration: frontend menu entry and list o... (#425154 - closed)
7. GAR integration: frontend artifact details page (#425157 - closed)
8. GAR integration documentation (#425158 - closed)