Don't check for xz on darwin

xz is not needed on darwin and checking for it generates a lot
of questions for new users that we could easily avoid.

ghcup

@@ -590,13 +590,20 @@ command_exists() {
 check_required_commands() {
     _missing_commands=
 
-    for com in "$@" awk uname basename tar xz gzip mktemp dirname ; do
+    for com in "$@" awk uname basename tar gzip mktemp dirname ; do
         command_exists "${com}" || {
             _missing_commands="${_missing_commands} ${com}"
         }
     done
     unset com
 
+    # darwin uses tar's built-in xz decompression
+    if test "${mydistro}" != "darwin"; then
+        command_exists xz || {
+            _missing_commands="${_missing_commands} xz"
+        }
+    fi
+
     if [ -n "${_missing_commands}" ] ; then
         printf "%s" "${_missing_commands}"
         unset _missing_commands