githubissues notifier - use single notifier for multiple repositories

Current configuration only allows a single repository
```
githubRepo: myuser/myrepo
```

In a larger scale where teams work on multiple repositories, it is undesirable to deploy multiple Cloud Run notifier for each individual repository (from both cost and maintenance perspective)

With a small tweak to use repository full name from Build's default substitutions value of `REPO_FULL_NAME`, we can deploy the notifier once and use it for multiple repositories.

See default substitutions here: https://cloud.google.com/build/docs/configuring-builds/substitute-variable-values#using_default_substitutions