R1:6df278bea84a

In "bin/ssh-auth", cache a structure instead of a flat file because paths may change at runtime

Summary:
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 Currently, `bin/ssh-auth` caches a flat file which includes the path to `bin/ssh-exec`, so this may fall out of date if `phabricator/` moves.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…