Upgrade test: started by hand only, and two checks that prove what they say

The workflow no longer runs for pull requests. It is started by hand,
and still posts to the pull request of the branch it was started on.
The fork fallback and the fork guard go with the trigger.

Two checks could pass without the thing they named being true:

The wallpaper check asked only whether the file the link points at
exists, so an update that swapped the user's picture for another would
pass. It now compares before and after: the same picture, whatever
format it is kept in, and present.

The failed-units check read an empty answer as nothing failed, which is
also what a service manager that cannot be asked gives. A query that
fails is now recorded as that, and fails the check.

README.md

@@ -280,7 +280,7 @@ The "Upgrade test" workflow does both steps from a release's ISO. Start it by ha
 gh workflow run upgrade-test.yml -f to=edge
 ```
 
-When the branch it ran on has a pull request, the result is posted there as one comment and updated in place on later runs.
+It only runs when started. When the branch it ran on has a pull request, the result is posted there as one comment and updated in place on later runs.
 
 ## Signing the ISO