Previously, for processes launched by doas the unwrapped doas binary preceded the
setuid-wrapped doas binary in PATH.
This caused error `doas: not installed setuid` when running doas from
processes launched by doas.
doas seems to short-circuit the PATH lookup when called like
`doas -u myuser doas -u myuser ...` so the error doesn't appear in this case.
`doas` is a lighter alternative to `sudo` that "provide[s] 95% of the
features of `sudo` with a fraction of the codebase" [1]. I prefer it to
`sudo`, so I figured I would add a NixOS module in order for it to be
easier to use. The module is based off of the existing `sudo` module.
[1] https://github.com/Duncaen/OpenDoas