Auto merge of #29712 - mrobinson:no-test-results-for-closed-prs, r=mukilan

Don't report test results for closed PRs

When doing a try run, bors will often push the last closed merge onto the branch before pushing the change to try. This means that test results get reported on closed PRs. There are two issues with this:

1. Doing too much work on the bots.
2. Extra results on closed PRs.

This changes fixes the second issue.

Fixes #29583.

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<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `___` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #29583

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- [x] These changes do not require tests because they fix a infrastructure issue.

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