Allow preserving the old working dir when setting up the indexer
Allow preserving the old working dir when setting up the indexer

When indexing mozilla-central, blowing away the working dir is
painful because it has to redownload the world which takes forever.
This allows devs (who know what they are doing) to run indexer-setup
with KEEP_WORKING=1 to keep the working directory instead of
blowing it away.

bill-mccloskey/searchfox,bill-mccloskey/searchfox,bill-mccloskey/searchfox,bill-mccloskey/searchfox,bill-mccloskey/searchfox,bill-mccloskey/searchfox