discover-image: Ignore sysupdate temporary files

Sysupdate temporary file names do not match their extension-release names. So
they will always fail. That makes enabling any other sysexts/confexts fail
which has catastrophic consequences. Unfortunately since 260, sysupdate
leaves temporary files for long time instead just while downloading. So
this kind of failure now happens much more often.

(cherry picked from commit 53b44ddfa7)

src/shared/discover-image.c

@@ -1110,6 +1110,10 @@ int image_discover(
                         if (dot_or_dot_dot(fname))
                                 continue;
 
+                        /* Ignore sysupdate temporary files */
+                        if (startswith(fname, ".sysupdate."))
+                                continue;
+
                         fname_path = path_join(search_path, fname);
                         if (!fname_path)
                                 return -ENOMEM;