Update workflow triggers to avoid duplicates

[azeey]

🦟 Bug fix

Summary

Pull requests created from a branch on gazebosim/gz-sim cause both the push and pull_request events to trigger a job.

Other than being a waste of resources, the duplicate jobs cause weird behaviors with codecov results. This PR updates the triggers so that the job runs only on pull_request. The push event should be triggered when we actually merge to the base branch.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

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[codecov]

Codecov Report

Merging #1988 (e12744d) into ign-gazebo3 (a6be730) will not change coverage.
The diff coverage is n/a.

❗ Current head e12744d differs from pull request most recent head 751126a. Consider uploading reports for the commit 751126a to get more accurate results

@@             Coverage Diff              @@
##           ign-gazebo3    #1988   +/-   ##
============================================
  Coverage        78.03%   78.03%           
============================================
  Files              255      255           
  Lines            15082    15082           
============================================
  Hits             11769    11769           
  Misses            3313     3313           

[mjcarroll]

There was a reason that we had both, but I don't recall what it was. Maybe something to do with running actions against PRs from forked repositories?

[azeey]

There was a reason that we had both, but I don't recall what it was. Maybe something to do with running actions against PRs from forked repositories?

PRs forked from repositories were only triggering the pull_request event anyway, so I don't think that would have been it.

From: https://github.com/gazebosim/gz-sim/actions/runs/4867734039

[mjcarroll]

Hmm, force pushes maybe?