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.
Using pkgs.lib on the spine of module evaluation is problematic
because the pkgs argument depends on the result of module
evaluation. To prevent an infinite recursion, pkgs and some of the
modules are evaluated twice, which is inefficient. Using ‘with lib’
prevents this problem.
