[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 when xv isn't installed. Check for this
scenario and fallback to using gzip-compressed tarballs.

Fixes nvm-sh#3034

nvm.sh

@@ -4325,7 +4325,11 @@ nvm_supports_xz() {
  if [ "_${NVM_OS}" = '_darwin' ]; then
  local MACOS_VERSION
  MACOS_VERSION="$(sw_vers -productVersion)"
- if nvm_version_greater "10.9.0" "${MACOS_VERSION}"; then
+ if tar --version | command grep -q GNU && ! command which xz >/dev/null 2>&1; then
+ # On macOS with GNU tar in use, and no xv on the path, xv-compressed
+ # tarballs aren't supported
+ return 1
+ elif nvm_version_greater "10.9.0" "${MACOS_VERSION}"; then
  # macOS 10.8 and earlier doesn't support extracting xz-compressed tarballs with tar
  return 1
  fi