R1:9b7d5b74d4a1

Purge `ssh-auth` key cache after trust/untrust

Summary: See PHI358. The `bin/almanac [un]trust-key` workflows don't properly purge the SSH key cache, but should.

Test Plan:
- Added key `ssh-rsa xyz` to a device.
- Used `bin/ssh-auth | grep xyz` to test for the presence of the key.
- Before patch: Saw it not present, trusted it, saw it still not present.
- After patch: Saw it not present, trusted it, saw it now present. Untrusted it, saw it no longer present.

Differential Revision: https://secure.