chore(ci): pin hashbrown for MSRV job #2912

Job	Run time
Verify hyper.h is up to date	29s
Check Style	3s
Test with Miri	2m 0s
Test C API (FFI)	59s
features	51s
Check deprecated on stable	19s
Check MSRV (1.63)	30s
Check deprecated on beta	27s
Test stable on ubuntu-latest	1m 40s
Test stable on windows-latest	3m 49s
Test stable on macOS-latest	2m 5s
Test beta on ubuntu-latest	1m 50s
Test beta on windows-latest	3m 22s
Test beta on macOS-latest	1m 48s
Test nightly on ubuntu-latest	1m 42s
Test nightly on windows-latest	3m 14s
Test nightly on macOS-latest	2m 8s
Build docs	21s
CI is green	0s
	27m 37s

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