`btrbk snapshot` connects to targets

Sorry if this is a duplicate - I tried to search but didn't see anything obvious.

When I run `btrbk snapshot`, it tries to connect to SSH targets, but my key has a passphrase because I want to automate snapshot creation, NOT resume.

I can't think of a valid reason for this behaviour. Surely snapshotting should be entirely local?

Reproduce `btrbk snapshot` with an SSH target and a passphrase-protected key, then trace how the snapshot command decides to connect to remote targets. Done means local snapshot creation no longer attempts an SSH connection, while the issue's resume behavior remains unaffected.

Written by the indexing model from the issue text.