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fix(run): `--target X` wrote X's build into the host's cache slot #1309

Job	Run time
toolchain: musl + llvm	1m 56s
toolchain: gcc (cold self-host)	1m 37s
integration: mcpp builds & runs xlings	1m 12s
build + unit tests (linux x86_64, self-host)	1m 12s
	5m 57s