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

src/sysupdate/sysupdate.c

@@ -1202,7 +1202,9 @@ static int context_install(
         }
 
         (void) sd_notifyf(/* unset_environment=*/ false,
-                          "STATUS=Installing '%s'.", us->version);
+                          "READY=1\n"
+                          "X_SYSUPDATE_VERSION=%s\n"
+                          "STATUS=Installing '%s'.", us->version, us->version);
 
         for (size_t i = 0; i < c->n_transfers; i++) {
                 Instance *inst = us->instances[i];