Re-enable SSH host key checking for all operations except when adding repo

[m3nu]

This limits accepting unknown SSH host keys to the inital repo setup. I.e. borg info and borg init. All other operations would fail if the original host key changed.

This is a trade-off between usability and security. Eventually we'd like to ask for the host key fingerprint when adding a new repo and verify it when adding.

See also: #714

[samuel-w]

I get this crash with a custom SSH key

Traceback (most recent call last):
  File "/home/user/Projects/vorta-manu/src/vorta/views/archive_tab.py", line 211, in list_action
    thread = BorgListRepoThread(params['cmd'], params, parent=self.app)
  File "/home/user/Projects/vorta-manu/src/vorta/borg/borg_thread.py", line 80, in __init__
    env['BORG_RSH'] += f' -i {ssh_key_path}'
KeyError: 'BORG_RSH'
Aborted

[m3nu]

Good find. Let me fix that.

[m3nu]

Should be fixed now, @samuel-w

[m3nu]

Thanks for reviewing and improving, @samuel-w 🙏