makechrootpkg: Fix unconditionally running namcap

Fixes regression in 2fd5931a8c

$run_namcap will always be set to ""
`if $not_a_var; then ...; fi` is always truthful when $not_a_var is
unset or equal to "" and the `then` clause will always be run.

I'm not sure why global state variables need to be cloned locally for
their sole explicit purpose.

But for now this patch implements the minimum necessary work to properly
pass the "do I want namcap" variable into prepare_chroot() according to
the current logic flow.
Note that I have still not thorougly tested makechrootpkg.

Signed-off-by: Eli Schwartz <eschwartz@archlinux.org>

makechrootpkg.in

@@ -409,7 +409,7 @@ main() {
 
 	download_sources "$copydir" "$makepkg_user"
 
-	prepare_chroot "$copydir" "$USER_HOME" "$keepbuilddir"
+	prepare_chroot "$copydir" "$USER_HOME" "$keepbuilddir" "$run_namcap"
 
 	if arch-nspawn "$copydir" \
 		--bind="$PWD:/startdir" \