chore(ci): pin hashbrown in MSRV job #2888

Job	Run time
Verify hyper.h is up to date	2m 14s
semver	2m 32s
Check Style	4s
features	33s
minimal-versions	57s
Test with Miri	2m 38s
Test C API (FFI)	42s
Check MSRV	2m 1s
Test stable on ubuntu-latest	1m 33s
udeps	22s
Test stable on windows-latest	3m 10s
Test stable on macOS-latest	1m 16s
Test beta on ubuntu-latest	1m 23s
Test beta on windows-latest	2m 22s
Test beta on macOS-latest	1m 25s
Test nightly on ubuntu-latest	1m 22s
Test nightly on windows-latest	2m 32s
Test nightly on macOS-latest	1m 25s
Check exposed types	58s
Build docs	24s
CI is green	1s
	29m 54s

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