[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.

test/mocks/uname_linux_aarch64

@@ -0,0 +1,5 @@
+if [ "_$1" = "_-m" ]; then
+  echo "aarch64"
+else
+  echo "Linux hostname 5.15.0-1 #1 SMP aarch64 aarch64 aarch64 GNU/Linux"
+fi

test/mocks/uname_linux_x86_64

@@ -0,0 +1,5 @@
+if [ "_$1" = "_-m" ]; then
+  echo "x86_64"
+else
+  echo "Linux hostname 5.15.0-1 #1 SMP x86_64 x86_64 x86_64 GNU/Linux"
+fi