feat: use github app

[fzipi]

After trying different permissions, looks like the only options for triggering another workflow are:

• using a PAT
• creating a new github app to trigger it

As using a PAT for this is a problem (mainly for me), I've created a new app with the sole purpose of triggering this result.

Also switched to $GITHUB_REPOSITORY_OWNER so forked repos won't try to trigger this action.

[dune73]

My condolences ...