makerepropkg: take makepkg.conf from devtools pkgdatadir location

This ensures we use the same configuration for reproducing packages as
we use for building them via devtools.
One example of why we care about this are the COMPRESS* settings that
may differ from the guest's pacman shipped makepkg.conf that affect the
reproducibility of packages.

makerepropkg.in

@@ -153,8 +153,7 @@ for fname in "${installed[@]}"; do
         exit 1
     fi
 done
-printf '%s\n' "${allpkgfiles[@]}" | mkarchroot -U "${archroot_args[@]}" "${buildroot}"/root - || exit 1
-
+printf '%s\n' "${allpkgfiles[@]}" | mkarchroot -M @pkgdatadir@/makepkg-x86_64.conf -U "${archroot_args[@]}" "${buildroot}"/root - || exit 1
 
 # use makechrootpkg to prep the build directory
 makechrootpkg -r "${buildroot}" -l "${chroot}" -- --packagelist || exit 1