CI: disable check-external-types errors until a new version is released #473

Job	Run time
Build documentation	20s
Validate external types appearing in public API	56s
Clippy	17s
Check (stable)	21s
Check (1.63.0)	1m 49s
Rustfmt	17s
no-std	21s
Check (nightly)	18s
Check semver compatibility	20s
Test suite (with features) (--features=default)	47s
Test suite (with features) (--all-features)	57s
Test suite (with features) (--features=bigint,serialize,debug)	44s
Test suite (with features) (--features=bigint,serialize,trace)	42s
	8m 9s

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