Touch-up CI to closer match Rustls CI #183

Job	Run time
Clippy	12s
Clippy (Nightly)	14s
Format	10s
Build+test (stable, ubuntu-20.04)	1m 2s
Check minimum versions of direct dependencies	17s
Build+test (beta, ubuntu-20.04)	1m 3s
msrv (ubuntu-20.04)	4m 18s
Check semver compatibility	8s
Build+test (nightly, ubuntu-20.04)	1m 4s
msrv (macos-latest)	6m 11s
Build+test (macos-latest, stable)	3m 32s
msrv (windows-latest)	5m 46s
Build+test (windows-latest, stable)	2m 8s
	26m 5s

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