Enhancement: Override the environment variable `RESTIC_HOST` with `--host`

When the environment variable `RESTIC_HOST` was set, snapshot listings and
other operations were always filtered to that host. Passing `--host` as an empty
string (`--host=""` or `--host=`) now overrides `RESTIC_HOST` and shows
snapshots from all hosts. The same override applies to commands that support
snapshot filters, including `snapshots`, `forget`, `find`, `stats`, `copy`,
`tag`, `repair snapshots`, `rewrite`, `mount`, `restore`, `dump`, and `ls`.

https://github.com/restic/restic/issues/5440
https://github.com/restic/restic/pull/5541
