Improve fork experience

[chapulina]

Our GitHub actions haven't been running correctly for forks. This should fix all of them:

CI

CI only on push means that PRs from forks aren't tested. Adding pull_request fixes forks, with the downside of triggering duplicated jobs for PRs coming from branches in this repo.

Also removed the codecov token for the Focal builds because we only need to upload coverage reports once, and this causes failures sometimes.

Labeler

Resolves gazebo-tooling/pr-collection-labeler#4.

Uses the new pull_request_target event.

Tested here: gazebosim/testing#14

Triage

Like above, uses the new pull_request_target event.

Tested here: gazebosim/testing#15

[codecov]

Codecov Report

Merging #165 into ign-rendering3 will not change coverage.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           ign-rendering3     #165   +/-   ##
===============================================
  Coverage           50.45%   50.45%           
===============================================
  Files                 129      129           
  Lines               11750    11750           
===============================================
  Hits                 5929     5929           
  Misses               5821     5821           

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