ExternalProject: Stop patch target from always appearing out-of-date

The skip-update target is always considered out-of-date.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always out of date too.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
1 file changed