ci: one pinned-mcpp bootstrap, not six drifting copies #567

Job	Run time
toolchain: gcc (cold self-host)	5m 45s
build + unit tests (linux x86_64, self-host)	6m 45s
toolchain: musl + llvm	4m 28s
integration: mcpp builds & runs xlings	6m 18s
	23m 16s

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