commands/pgp: Unregister the "check_signatures" hooks on module unload

If the hooks are not removed they can be called after the module has
been unloaded leading to an use-after-free.

Fixes: CVE-2025-0622

Reported-by: B Horn <b@horn.uk>
Signed-off-by: B Horn <b@horn.uk>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

grub-core/commands/pgp.c

@@ -1010,6 +1010,8 @@ GRUB_MOD_INIT(pgp)
 
 GRUB_MOD_FINI(pgp)
 {
+  grub_register_variable_hook ("check_signatures", NULL, NULL);
+  grub_env_unset ("check_signatures");
   grub_verifier_unregister (&grub_pubkey_verifier);
   grub_unregister_extcmd (cmd);
   grub_unregister_extcmd (cmd_trust);