sysupdate: Emit READY=1 status when installing

`READY=1` is already correctly emitted when acquiring an update, but was
forgotten to be emitted when subsequently installing that update.

That meant that the state tracking code in `sysupdated` and hence
`updatectl` could not properly report the progress of the install
operation, and hence printed “Already up-to-date” after a successful
update installation, rather than “Done”.

Add a test to catch this in future.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

Fixes: https://github.com/systemd/systemd/issues/41502

test/units/TEST-72-SYSUPDATE.sh

@@ -373,7 +373,9 @@ EOF
     if $have_updatectl; then
         systemctl start systemd-sysupdated
         "$SYSUPDATE" --verify=no check-new
-        updatectl update
+        updatectl update |& tee "$WORKDIR"/updatectl-update-6
+        grep "Done" "$WORKDIR"/updatectl-update-6
+        (! grep "Already up-to-date" "$WORKDIR"/updatectl-update-6)
     else
         # If no updatectl, gracefully fall back to systemd-sysupdate
         update_now "$update_type"