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 #189 into ign-transport8 will increase coverage by 0.12%.
The diff coverage is n/a.

@@                Coverage Diff                 @@
##           ign-transport8     #189      +/-   ##
==================================================
+ Coverage           83.41%   83.54%   +0.12%     
==================================================
  Files                  49       49              
  Lines                4920     4849      -71     
==================================================
- Hits                 4104     4051      -53     
+ Misses                816      798      -18     

Impacted Files	Coverage Δ
include/ignition/transport/Discovery.hh	87.65% <0.00%> (-0.06%)	⬇️
src/Node.cc	92.32% <0.00%> (-0.03%)	⬇️
src/NodeShared.cc	78.38% <0.00%> (+0.43%)	⬆️

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