Enhancement: Allow overriding RESTIC_HOST environment variable with --host flag

When the `RESTIC_HOST` environment variable was set, there was no way to list or
operate on snapshots from all hosts, as the environment variable would always
filter to that specific host. Restic now allows overriding `RESTIC_HOST` by
explicitly providing the `--host` flag with an empty string (e.g., `--host=""` or
`--host=`), which will show snapshots from all hosts. This works for all commands
that support snapshot filtering: `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