apparmor: replace misleading 'scrubbing environment' phrase in debug print

The wording of 'scrubbing environment' implied that all environment
variables would be removed, when instead secure-execution mode only
removes a small number of environment variables. This patch updates the
wording to describe what actually occurs instead: setting AT_SECURE for
ld.so's secure-execution mode.

Link: https://gitlab.com/apparmor/apparmor/-/merge_requests/1315

 is a
merge request that does similar updating for apparmor userspace.

Signed-off-by: Ryan Lee <ryan.lee@canonical.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>