Enhancement: Make `--host` override environment variable `RESTIC_HOST`

Previously, 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 other 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
