offload-build: take makepkg.conf from devtools for appropriate packagelist

This fixes an issue with the usage of makepkg --packagelist to get the
produced artifacts filenames according to the PKGEXT used in devtools'
makepkg.conf instead of the one defined in pacman.

One goal we want to preserve is that devtools configuration should be
self contained and not require any editing of non owned files like
the host /etc/makepkg.conf to produce expected results.

Additionally modify the archbuild_cmd override for multilib builds to
use an independent variable and not fiddle with the actual arch
variable to select the appropriate cmd.

offload-build.in

@@ -74,11 +74,12 @@ while (( $# )); do
 done
 
 # multilib must be handled specially
+archbuild_arch="${arch}"
 if [[ $repo = multilib* ]]; then
-    arch=
+    archbuild_arch=
 fi
 
-archbuild_cmd=("${repo}${arch:+-$arch}-build" "$@")
+archbuild_cmd=("${repo}${archbuild_arch:+-$archbuild_arch}-build" "$@")
 
 trap 'rm -rf $SRCPKGDEST' EXIT INT TERM QUIT
 
@@ -108,7 +109,11 @@ mapfile -t files < <(
                 printf "%s\n" "" "-> build complete" &&
                 printf "\t%s\n" "$temp"/*
             } >&2 &&
-            makepkg --packagelist
+            makepkg_config="/usr/share/devtools/makepkg-'"${arch}"'.conf" &&
+            if [[ -f /usr/share/devtools/makepkg-'"${repo}"'-'"${arch}"'.conf ]]; then
+                makepkg_config="/usr/share/devtools/makepkg-'"${repo}"'-'"${arch}"'.conf"
+            fi &&
+            makepkg --config "${makepkg_config}" --packagelist
 ')