meson: prevent finding sed(1) in a loop

meson: prevent finding sed(1) in a loop

We're searching for the sed(1) executable in a loop, which will make us
try to find it multiple times. Starting with the preceding commit we
already declare a variable for that program in the top-level build file.
Use it so that we only need to search for the program once.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>