fix: switch to statically linked `coreutils-*-linux-musl`
The coreutils-*-linux-gnu binaries have dynamic link to glibc.
Switching to the musl variants that are statically linked.
Leaving in the download targets for the gnu variants as we
could register them with the gnu constraint from toolchain_utils
but there is not much point, because the musl variant will
always work.
Closes #1 (closed)
Edited by Matthew Clarkson