[trusted-contributions-gcf]: consider adding a /gcbrun comment

[coryan]

Is your feature request related to a problem? Please describe.

When renovatebot creates a PR I have to manually start the GCB (Google Cloud Build) builds, while the Kokoro builds start automatically.

Describe the solution you'd like

The trusted-contributions-gcf bot needs to insert a comment saying /gcbrun.

Describe alternatives you've considered

AFAIK this is it, there is no other way to trigger GCB builds, but I would be happy to hear otherwise.

Additional context

This could be a good example:

googleapis/google-cloud-cpp#6357

/cc: @devjgm in case he has a better idea.

[devjgm]

It may be possible to have our google-cloud-cpp-bot detect that a PR is coming from renovatebot and then add a /gcbrun comment. Not sure if that's secure enough though.

I don't know what trusted-contributions-gcf is or if that's a better idea.