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

every time we update builder. When get-upstream-dists' codepath to
update from apt-cacher-ng cache is triggered we might get old(er) files,
but hopefully it won't cause problems.

As such, I'll move it to the tools directory with the other
update script(s). It also requires the script look in a new location for
its config file - thats what this change provides.