IssueTracker

Currently, the "$_PR_NUMBER" Cloud Build substitution is only available for GitHub pull request triggers. This limits the ability to automate build processes based on pull request numbers for GitLab projects. The goal is that the "iid" of a GitLab pull request can be accessed through this substitution.

Making this substitution available for GitLab expands the flexibility of the Cloud Build service to create automations and improve integration with GitLab. User will also be able to migrate between the two systems more easily.