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

Restic commands that operate on snapshots, such as `restic backup` and
`restic snapshots`, support the `--host` option to specify the hostname
for grouping snapshots.

Such commands now also support specifying the hostname via the environment
variable `RESTIC_HOST`. Note that `--host` still takes precedence over the
environment variable.

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