selftests/landlock: Fix error message

The global variable errno may not be set in test_execute().  Do not use
it in related error message.

Cc: Günther Noack <gnoack@google.com>
Fixes: e1199815 ("selftests/landlock: Add user space tests")
Link: https://lore.kernel.org/r/20250108154338.1129069-21-mic@digikod.net


Signed-off-by: Mickaël Salaün <mic@digikod.net>