Don't try to determine where musl is determined based on the target vendor being alpine or not

[bjorn3]

This is wrong when trying to use a rustup installed rustc on alpine or when globally installing musl on a non-alpine system. I would suggest always assuming that musl is globally installed and if this isn't the case ask the user to manually pass in the required -Clink-args required to make the linker find musl. This is necessary anyway for other crates like libc to work and it will allow it to work with the musl copy shipped with rustc itself when statically linking. For determining if static or dynamic linking should be used, checking the presence of the crt-static target feature works much better as this is what actually determines if libc will be statically linked or not.