Change into the builder root before sourcing config.

I forgot config is sourced relative to $PWD, not its starting dir
(probably a bug in that code tbh...)

tools/get-upstream-dists

@@ -18,7 +18,8 @@
 #
 
 set -ex
-. `dirname $0`/../config
+cd `dirname $0`/../
+. config
 
 # Create our working directories if they don't exist
 [ ! -d $MIRRORLOCAL/dists/ ] && mkdir -p $MIRRORLOCAL/dists/