`nvm_get_arch`: skip the smartos arm64/armv8l cases, which have no uname mock and fall through to the real host (adding a musl suffix on Alpine).
`nvm_get_arch_unofficial`: skip the glibc chroot (fixed /lib64 loader, sudo); the musl mapping is already covered by the `nvm_get_arch alpine` test.
`nvm_install_no_progress_bar`: skip below the musl floor, since v0.12.18 has no musl binary and hardcodes the glibc tarball URL.
nvm.sh uses `NVM_SCRIPT_SOURCE="$_"` to detect its source location.
Adding `: nvm.sh` before each source line ensures `$_` is set correctly, preventing breakage when the previous command (e.g., `set -ex`) overwrites it.