ci: Improve times by splitting stable test runs #583

Job	Run time
Check for changes in Rust files	6s
Check for changes in Rust files	5s
tests (Rust stable, all features)	1m 34s
tests (Rust stable, all features)	51s
check	1m 0s
check	45s
tests (Rust 1.75)	1m 57s
tests (Rust 1.75)	1m 39s
benches	1m 37s
benches	1m 23s
tests (Rust beta)	1m 53s
tests (Rust beta)	1m 25s
tests (Rust stable, no features)	1m 27s
tests (Rust stable, no features)	1m 6s
tests (Rust nightly)	2m 32s
tests (Rust nightly)	1m 38s
Required checks 🦀	1s
Required checks 🦀	1s
coverage	3m 6s
coverage	2m 53s
	26m 59s

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