[Fix] gracefully handle GNU tar on MacOS during npm install

GNU tar doesn't support using the -J flag required to unpack
xz-compressed tarballs. This change means that anyone using
GNU tar on MacOS will instead use gz-compressed tarballs.

Fixes nvm-sh#3034

nvm.sh

@@ -4325,7 +4325,10 @@ nvm_supports_xz() {
   if [ "_${NVM_OS}" = '_darwin' ]; then
     local MACOS_VERSION
     MACOS_VERSION="$(sw_vers -productVersion)"
-    if nvm_version_greater "10.9.0" "${MACOS_VERSION}"; then
+    if tar --version | command grep -q GNU; then
+      # On macOS with GNU tar in use xv-compressed tarballs aren't supported
+      return 1
+    elif nvm_version_greater "10.9.0" "${MACOS_VERSION}"; then
       # macOS 10.8 and earlier doesn't support extracting xz-compressed tarballs with tar
       return 1
     fi