Make it work with PRs from forks

[chapulina]

The GitHub token provided with PRs from forks doesn't have enough permissions for labeling. The actions API also doesn't allow us injecting a valid token from the secrets.

Until recently, I thought we needed to use a polling strategy like on gazebosim/docs#75 to assign these labels. It's less immediate and uses more resources, but it seemed to be what people were doing everywhere. Until recently GitHub announced the new pull_request_target event that should be helpful here.

For completeness, I started working on a polling solution a while back but never finished:

master...chapulina/periodic

[chapulina]

The pull_request_target even works like a charm. Tested here: gazebosim/testing#14

Now I need to open PRs for alllll repositories to use it 😬