Pin the MCP conformance tester package (#1176) #1724

Job	Run time
build (macos-latest, Release)	8m 6s
build (windows-latest, Debug)	9m 36s
build (windows-latest, Release)	10m 13s
build (ubuntu-latest, Debug)	6m 56s
build (ubuntu-latest, Release)	11m 6s
build (macos-latest, Debug)	9m 36s
publish-coverage	0s
	55m 33s

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