nix-env: Minor change to '--delete-generations Nd' semantics

The option '--delete-generations Nd' deletes all generations older than N
days. However, most likely the user does not want to delete the
generation that was active N days ago.

For example, say that you have these 3 generations:

```
1: <30 days ago>
2: <15 days ago>
3: <1 hour ago>
```

If you do --delete-generations 7d (say, as part of a cron job), most
likely you still want to keep generation 2, i.e. the generation that was
active 7 days ago (and for most of the past 7 days, in fact).

This patch fixes this issue. Note that this also affects
'nix-collect-garbage --delete-older-than Nd'.

Thanks to @roconnor for noticing the issue!