Enhancement: Allow setting REST password and username via environment variables

Previously, it was only possible to specify the REST-server username and
password in the repository URL, or by using the `--repository-file` option.
This meant it was not possible to use authentication in contexts where the
repository URL is stored in publicly accessible way.

Restic now allows setting the username and password using the
`RESTIC_REST_USERNAME` and `RESTIC_REST_PASSWORD` variables.

https://github.com/restic/restic/pull/4480
