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 using the `--repository-file` option. This
meant it was not possible to use authentication in contexts where the repository
URL is public and parts of it are templated by other software. 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