Fix handling of syscalls only allowed by --devel

This was incorrectly looking at errno instead of -r.

Fixes: 0b38b0f0 "run: Handle unknown syscalls as intended"
Signed-off-by: Simon McVittie <smcv@collabora.com>
(cherry picked from commit 3fc8c672676ae016f8e7cc90481b2feecbad9861)