Return NOT_FOUND from the set_cmnd_path() stub since we don't set user_cmnd.

The purpose of set_cmnd_path() is to reset user_cmnd based on a new
runchroot.  For the stub version we don't modify user_cmnd and so
must not return a status of FOUND.
Fixes oss-fuzz issue #31250 which only affected the fuzzer and not sudo.

plugins/sudoers/stubs.c

@@ -84,7 +84,8 @@ get_interfaces(void)
 int
 set_cmnd_path(const char *runchroot)
 {
-    return FOUND;
+    /* Cannot return FOUND without also setting user_cmnd to a new value. */
+    return NOT_FOUND;
 }
 
 /* STUB */