Actions:Allow type check to run at PR level

[marcotc]

All external PRs are stuck because "Check types" does run for external contributors, not even after manual approval (see #2928, for example):

Interestingly CodeQL does work correctly, running on approval:

My research was not 100% conclusive, but I believe that the on trigger is the reason.

This PR change the on trigger to look more similar to CodeQL's, specifically regarding having an explicit pull_request trigger.

[codecov-commenter]

Codecov Report

Merging #2929 (590789f) into master (b5eee29) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2929      +/-   ##
==========================================
- Coverage   97.96%   97.96%   -0.01%     
==========================================
  Files        1282     1282              
  Lines       70682    70681       -1     
  Branches     3249     3249              
==========================================
- Hits        69241    69240       -1     
  Misses       1441     1441              

see 3 files with indirect coverage changes

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[ivoanjo]

Seems reasonable! Thanks for improving the experience for contributors :)

LGTM 👍