CI: display toolchain version (cosmetic) #183

Job	Run time
prek	49s
Check (stable)	9s
Check (nightly)	16s
Rustfmt	11s
Check (1.63.0)	1m 34s
Test Suite (--features=serialize)	24s
Test Suite (--features=integers)	23s
Build documentation	18s
Clippy	15s
Check semver compatibility	23s
Validate external types appearing in public API	17s
Test Suite (--no-default-features)	9s
Test Suite (--all-features)	20s
	5m 28s

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