nixos/modules/services/scheduling/fcron: add setuid to fcrontab

Without fcrontab being setuid, every attempt by an user in the fcron
group to edit their own crontab (via `fcrontab -e`) results in the
following error:

```
2018-05-06 11:29:07 ERROR could not change euid to 273: Operation not permitted
2018-05-06 11:29:07 ERROR fcron child aborted: this does not affect the main fcron daemon, but this may prevent a job from being run or an email from being sent.
```

Adding setuid by hand has resolved this issue and aligns with the way
fcrontab is installed on other distributions.