ci: Only build Fx for PRs #2757

Job	Run time
Build & test (ubuntu-latest, 1.76.0, debug)	11m 25s
Build & test (ubuntu-latest, stable, debug)	12m 13s
Build & test (ubuntu-latest, nightly, debug)	12m 39s
Build & test (macos-14, 1.76.0, debug)	11m 11s
Build & test (macos-14, stable, debug)	10m 35s
Build & test (macos-14, nightly, debug)	8m 41s
Build & test (windows-latest, 1.76.0, debug)	20m 45s
Build & test (windows-latest, stable, debug)	22m 0s
Build & test (windows-latest, nightly, debug)	21m 54s
Build & test (ubuntu-latest, stable, release)	17m 37s
Benchmark / Benchmark	25m 55s
	2h 54m 55s

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