[Fix] support arm64-musl on Alpine Linux

Node.js unofficial-builds publishes linux-arm64-musl binaries since v20.20.1 / v22.21.1 / v24+.
Map NVM_ARCH=arm64 to arm64-musl on Alpine so `nvm install`
resolves to the correct tarball instead of the glibc-linked arm64 build that won't run against musl.

[Tests] `nvm_get_arch`: expect arm64-musl on Alpine

Co-authored-by: Jesse Zhu <jesse@itjesse.com>
Co-authored-by: Jordan Harband <ljharb@gmail.com>

nvm.sh

@@ -2252,9 +2252,11 @@ nvm_get_arch() {
   fi
 
   if [ -f "/etc/alpine-release" ]; then
-    # Alpine Linux uses musl libc; only x64-musl binaries are available
+    # Alpine Linux uses musl libc; map to musl variants where available
+    # See https://unofficial-builds.nodejs.org/download/release/
     case "${NVM_ARCH}" in
       x64) NVM_ARCH=x64-musl ;;
+      arm64) NVM_ARCH=arm64-musl ;;
     esac
   fi