Merge pull request #308594 from Artturin/grubsetplat

grub2_xen: set to `x86_64-linux` and `i686-linux` only

pkgs/tools/misc/grub/default.nix

···
 
     license = licenses.gpl3Plus;
 
-    platforms = platforms.gnu ++ platforms.linux;
+    platforms = if xenSupport then [ "x86_64-linux" "i686-linux" ] else platforms.gnu ++ platforms.linux;
 
     maintainers = [ maintainers.samueldr ];
   };