bin/n: Update can_use_xz() for macOS 11 (#624)

macOS 11 has been announced, and is expected to continue to support xz

bin/n

@@ -824,9 +824,14 @@ function can_use_xz() {
   if [[ "${uname_s}" = "Linux" ]] && command -v xz &> /dev/null ; then
     # tar on linux is likely to support xz if it is available as a command
     return 0
-  elif [[ "${uname_s}" = "Darwin" && "$(sw_vers -productVersion | cut -d '.' -f 2)" -gt "8" ]]; then
-    # tar on recent Darwin has xz support built-in
-    return 0
+  elif [[ "${uname_s}" = "Darwin" ]]; then
+    local macos_version="$(sw_vers -productVersion)"
+    local macos_major_version="$(echo ${macos_version} | cut -d '.' -f 1)"
+    local macos_minor_version="$(echo ${macos_version} | cut -d '.' -f 2)"
+    if [[ "${macos_major_version}" -gt 10 || "${macos_minor_version}" -gt 8 ]]; then
+      # tar on recent Darwin has xz support built-in
+      return 0
+    fi
   fi
   return 2 # not supported
 }