Bugfix: restic generate, help and self-update no longer check passwords

The commands `restic cache`, `generate`, `help` and `self-update` don't need
passwords, but they previously did run the RESTIC_PASSWORD_COMMAND (if set in
the environment), prompting users to authenticate for no reason. They now skip
running the password command.

https://github.com/restic/restic/issues/2951
https://github.com/restic/restic/pull/2987