[Fix] `nvm_get_arch`: only apply musl suffix on x64 Alpine

Alpine detection unconditionally set `x64-musl` regardless of actual architecture, which would be incorrect on ARM-based Alpine containers.

Bug introduced in https://github.com/nvm-sh/nvm/commit/ef7fc2f2c06ad75fe7fbabf28d427561ae7b007d / #3212.
Fixes #3616.