Handle SSH generated keys in Conduit passphrase.query

Summary: Fixes T6601

Test Plan: echo '{"ids":["71"],"needPublicKeys":true,"needSecrets":false}' | arc call-conduit passphrase.query

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley

Maniphest Tasks: T6601

Differential Revision: https://secure.phabricator.com/D10885