ExternalProject: Stop patch target from always appearing out-of-date

The skip-update target is always considered out-of-date. The change in
7249ba9677 (ExternalProject: Enforce that patch depends on update, 2020-04-03)
made the patch target depend on skip-update, which in turn made it
always out of date too. The patch command should only be re-run if the download
needs to be performed again where updates are disconnected.

Fixes: #21086

Modules/ExternalProject.cmake

@@ -2801,9 +2801,9 @@ function(_ep_add_patch_command name)
 
   _ep_get_update_disconnected(update_disconnected ${name})
   if(update_disconnected)
-    set(update_dep skip-update)
+    set(patch_dep download)
   else()
-    set(update_dep update)
+    set(patch_dep update)
   endif()
 
   set(__cmdQuoted)
@@ -2814,7 +2814,7 @@ function(_ep_add_patch_command name)
     ExternalProject_Add_Step(${name} patch
       COMMAND ${__cmdQuoted}
       WORKING_DIRECTORY \${work_dir}
-      DEPENDEES download \${update_dep}
+      DEPENDEES \${patch_dep}
       ${log}
       )"
   )