See https://discourse.phabricator-community.org/t/possible-arcanist-bug-upon-install/3603. See PHI1663.
In D21030, I added Conduit code to the new Toolsets runtime, but it may fall back and attempt to access an older piece of configuration called "default" in some cases. This piece of configuration is (at least currently) not formally defined in the new runtime (and I intend to deprecate/remove it eventually), so this call fails.