Fix path to the new build system, now called Hadrian.
Summary: The new Shake-based build system has been given a name -- Hadrian, and now lives in /hadrian directory. This fixes the path to the configuration file to be populated by the configure script. Test Plan: Run Hadrian build. Reviewers: thomie, bgamari, hvr, austin Reviewed By: austin Subscribers: erikd Differential Revision: https://phabricator.haskell.org/D2153