Revert "ci: temp. pin nightly to avoid ICE" #465

Job	Run time
build-windows	9m 7s
Validate external types appearing in public API	1m 27s
Build rustls-cert-gen as a standalone package	26s
build (macOS-latest, stable)	35s
build (ubuntu-latest, stable)	47s
Measure coverage	5m 37s
Format & clippy	5m 2s
build (ubuntu-latest, beta)	5m 25s
Documentation (stable)	53s
Documentation (nightly)	59s
build (ubuntu-latest, nightly)	5m 28s
Check semver compatibility	50s
build (ubuntu-latest, stable 7 months ago)	46s
	37m 22s

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