Enhancement: Allow specifying `--host` via environment variable

Restic commands that operate on snapshots, such as `restic backup` and
`restic snapshots`, support the `--host` flag to specify the hostname for
grouoping snapshots. They now permit selecting the hostname via the
environment variable `RESTIC_HOST`. `--host` still takes precedence over the
environment variable.

https://github.com/restic/restic/issues/4733
https://github.com/restic/restic/pull/4734