Allow testing workflow changes in dev from repositories within the grafana org

Currently, the WIF rules we have for accessing the resources in Google Cloud (GCS, SA) are locked down only to the main branch of this repository, making it hard to test PRs before merging them to main.

The only exception is for the testing repo, but it can only be accessed by the plugins-platform team.

It would be nice to add the ability to access to the dev GCS/SA from any branch of this repo, making it easier to test changes to the GitHub workflows coming from other teams (not plugins-platform).

This should only be allowed from repositories within the Grafana org, of course.