makechrootpkg: verify sources before preparing chroot

This is needed in order to use GPG's auto-key-retrieve keyserver option,
otherwise the keyring will get copied to the chroot before the required
keys are retrieved during 'makepkg --verifysource'.

makechrootpkg.in

@@ -384,10 +384,10 @@ $update_first && arch-nspawn "$copydir" \
 
 [[ -n ${install_pkgs[*]} ]] && install_packages
 
-prepare_chroot
-
 download_sources
 
+prepare_chroot
+
 if arch-nspawn "$copydir" \
 	--bind-ro="$PWD:/startdir_host" \
 	--bind-ro="$SRCDEST:/srcdest_host" \