diagnostic: fix XDG_DATA_DIRS check

1. Pass `XDG_DATA_DIRS` through so that it can be checked.
2. Promote the check to cross-platform status, as an incorrect `XDG_DATA_DIRS` also affects macOS formulae.

Library/Homebrew/diagnostic.rb

@@ -341,6 +341,19 @@ def check_tmpdir_sticky_bit
       end
       alias generic_check_tmpdir_sticky_bit check_tmpdir_sticky_bit
 
+      def check_xdg_data_dirs
+        xdg_data_dirs = ENV.fetch("HOMEBREW_XDG_DATA_DIRS", nil)
+        return if xdg_data_dirs.blank? || xdg_data_dirs.split("/").include?(HOMEBREW_PREFIX/"share")
+
+        <<~EOS
+          Homebrew's share was not found in your XDG_DATA_DIRS but you have
+          this variable set to include other locations.
+          Some programs like `vapigen` may not work correctly.
+          Consider adding Homebrew's share directory to XDG_DATA_DIRS like so:
+            echo 'export XDG_DATA_DIRS="#{HOMEBREW_PREFIX}/share:$XDG_DATA_DIRS"' >> #{Utils::Shell.profile}
+        EOS
+      end
+
       def check_exist_directories
         return if HOMEBREW_PREFIX.writable?
 

Library/Homebrew/extend/os/linux/diagnostic.rb

@@ -59,19 +59,6 @@ def check_tmpdir_executable
         f&.unlink
       end
 
-      def check_xdg_data_dirs
-        xdg_data_dirs = ENV.fetch("XDG_DATA_DIRS", nil)
-        return if xdg_data_dirs.blank? || xdg_data_dirs.split("/").include?(HOMEBREW_PREFIX/"share")
-
-        <<~EOS
-          Homebrew's share was not found in your XDG_DATA_DIRS but you have
-          this variable set to include other locations.
-          Some programs like `vapigen` may not work correctly.
-          Consider adding Homebrew's share directory to XDG_DATA_DIRS like so:
-            echo 'export XDG_DATA_DIRS="#{HOMEBREW_PREFIX}/share:$XDG_DATA_DIRS"' >> #{Utils::Shell.profile}
-        EOS
-      end
-
       def check_umask_not_zero
         return unless File.umask.zero?
 

bin/brew

@@ -185,6 +185,7 @@ USED_BY_HOMEBREW_VARS=(
   SUDO_USER
   TMUX
   XDG_CACHE_HOME
+  XDG_DATA_DIRS
   XDG_RUNTIME_DIR
   ZDOTDIR
 )