sysupdate: change status once operation has completed

Otherwise after the service exits it will still show
"Installing 'foobar'" as the status, which is confusing

src/sysupdate/sysupdate.c

@@ -1088,6 +1088,9 @@ static int context_apply(
 
         log_info("%s Successfully installed update '%s'.", glyph(GLYPH_SPARKLES), us->version);
 
+        (void) sd_notifyf(/* unset_environment=*/ false,
+                          "STATUS=Installed '%s'.", us->version);
+
         if (ret_applied)
                 *ret_applied = us;