Bugfix: Make `key list` command honor `--no-lock`

The `key list` command now supports the `--no-lock` options. This allows
determining which keys a repo can be accessed by without the need for having
write access (e.g., read-only sftp access, filesystem snapshot).

https://github.com/restic/restic/issues/4513
https://github.com/restic/restic/pull/4514
