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Improve `pr-checks.yml` triggers and conditions #4930

Job	Run time
Rust analysis (ubuntu-latest, stable-v2.20.7)	1m 10s
Rust analysis (ubuntu-latest, stable-v2.22.1)	1m 41s
Rust analysis (ubuntu-latest, default)	1m 38s
Rust analysis (ubuntu-latest, nightly-latest)	1m 35s
Rust analysis (ubuntu-latest, linked)	1m 35s
	7m 39s