Fix check-man for newer man-db

Again, check-man :)

Commit 5fa16422 removed LC_ALL=C, since that breaks the check.
However, with no LANG/LC_* variables, man-db is still broken.

We import the new lintian behaviour, i.e. LANG=C (which seems to differ
from LC_ALL=C, even with empty environment). I'm not sure of the
difference, though.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>