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>

Documentation/howto/meson.build

@@ -41,7 +41,7 @@ custom_target(
 foreach howto : howto_sources
   howto_stripped = custom_target(
     command: [
-      find_program('sed'),
+      sed,
       '-e',
       '1,/^$/d',
       '@INPUT@',