Fixes T12397. Ref T13164. See PHI801.
Several installs have hit various use cases where the path on disk where Phabricator lives changes at runtime. Currently, bin/ssh-auth caches a flat file which includes the path to bin/ssh-exec, so this may fall out of date if phabricator/ moves.
These use cases have varying strengths of legitimacy, but "we're migrating to a new set of hosts and the pool is half old machines and half new machines" seems reasonably compelling and not a problem entirely of one's own making.