core/unit: do not use unit path cache in unit_need_daemon_reload()

When unit_need_daemon_reload() calls unit_find_dropin_paths() to check
for new drop-in configs, the manager's unit path cache is used to limit
which directories are considered. If a new drop-in directory is created,
it may not be in the unit path cache, and hence unit_need_daemon_reload()
may return false, despite a new drop-in being present. However, if a
unit path cache is not given to unit_file_find_dropin_paths() at all,
then it behaves as if the target path was found in the unit path cache.

So, to fix this, adapt unit_find_dropin_paths() to take a boolean
argument indicating whether or not to pass along the unit path cache.
Set this to false in unit_need_daemon_reload().

Fixes #31752

src/core/unit.c

@@ -3809,7 +3809,7 @@ bool unit_need_daemon_reload(Unit *u) {
         if (u->load_state == UNIT_LOADED) {
                 _cleanup_strv_free_ char **dropins = NULL;
 
-                (void) unit_find_dropin_paths(u, &dropins);
+                (void) unit_find_dropin_paths(u, /* use_unit_path_cache = */ false, &dropins);
 
                 if (!strv_equal(u->dropin_paths, dropins))
                         return true;