Allow stylo to do lazier stylist rebuilds
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- [ ] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [ ] These changes fix https://bugzilla.mozilla.org/show_bug.cgi?id=1361843 (github issue number if applicable).
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- [ ] There are tests for these changes OR
- [X] These changes do not require tests because it's optimization, which should have no observable behavior difference.
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