[Robustness] nvm_get_arch: only apply the Alpine musl suffix when the OS is linux

The `/etc/alpine-release` check applied the -musl suffix regardless of the resolved OS.
In practice Alpine is always linux, so this is behavior-neutral, but it also makes the `nvm_get_arch` unit test hermetic: its mocked smartos/osx cases no longer pick up a real host's Alpine marker.

nvm.sh

@@ -2251,7 +2251,7 @@ nvm_get_arch() {
     HOST_ARCH=armv7l
   fi
 
-  if [ -f "/etc/alpine-release" ]; then
+  if [ -f "/etc/alpine-release" ] && [ "_${NVM_OS}" = "_linux" ]; then
     # Alpine Linux uses musl libc; map to musl variants where available
     # See https://unofficial-builds.nodejs.org/download/release/
     case "${NVM_ARCH}" in