R1:57f1ab705e28

Correct private key permissions before extracting public key in bin/almanac register

Summary: `ssh-keygen` declines to run on a too-public key. Write the correctly-restricted key a little earlier in the workflow.

Test Plan:
```
epriestley@orbital ~/dev/phabricator $ chmod 644 ~/dev/core/conf/keys/daemon.key
epriestley@orbital ~/dev/phabricator $ ./bin/almanac register --private-key ~/dev/core/conf/keys/daemon.key --identify-as local.phacility.net --device daemon.phacility.net --force --allow-key-reuse…