ci: update cargo-check-external-types toolchain, fix build #1366

Job	Run time
Documentation (stable)	56s
Documentation (nightly)	1m 0s
Build rustls-cert-gen as a standalone package	28s
build-windows	3m 27s
Validate external types appearing in public API	1m 28s
build (macOS-latest, stable)	7s
Measure coverage	1m 8s
build (ubuntu-latest, stable)	7s
Format & clippy	3m 41s
build (ubuntu-latest, beta)	6s
Check semver compatibility	57s
build (ubuntu-latest, nightly)	3s
build (ubuntu-latest, stable 7 months ago)	4s
	13m 32s

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