nvm.sh: Fall back to "xz on PATH" check

Also refactor, because the fallback check now uses the test
Linux had been using.

Author: DeeDeeG

nvm.sh

@@ -3693,12 +3693,7 @@ nvm_supports_xz() {
 
   local NVM_OS
   NVM_OS="$(nvm_get_os)"
-  if [ "_${NVM_OS}" = '_linux' ]; then
-    if ! command which xz >/dev/null 2>&1; then
-      # Linux without xz on the PATH doesn't support extracting xz-compressed tarballs with tar
-      return 1
-    fi
-  elif [ "_${NVM_OS}" = '_darwin' ]; then
+  if [ "_${NVM_OS}" = '_darwin' ]; then
     local MACOS_VERSION
     MACOS_VERSION="$(sw_vers -productVersion)"
     if nvm_version_greater "10.9.0" "${MACOS_VERSION}"; then
@@ -3711,8 +3706,11 @@ nvm_supports_xz() {
       return 1
     fi
   else
-    # Conservatively assume other operating systems don't support extracting xz-compressed tarballs with tar
-    return 1
+    if ! command which xz >/dev/null 2>&1; then
+      # Most OSes without xz on the PATH don't support extracting xz-compressed tarballs with tar
+      # (Should correctly handle Linux, SmartOS, maybe more)
+      return 1
+    fi
   fi
 
   # all node versions v4.0.0 and later have xz