mk_cmds: don't use explicit pathname for sed

mk_cmds: don't use explicit pathname for sed

$AWK doesn't use an explicit pathname, and it's perfectly fine to
assume that awk and sed are in the user's PATH.  The problem with
using an explicit pathname is that Debian currently allows merged and
non-merged /usr.  Avoid using an explicit pathname to prevent
potential problems.

Addresses-Debian-Bug: #914087
Signed-off-by: Theodore Ts'o <tytso@mit.edu>