hoogle service: fixups

Basic hardening
- Run as nobody:nogroup with a private /tmp, /home & /run/user
- Create working directory under /run (hoogle insists on writing to cwd
  and otherwise returns "something went wrong" to every query)

Option tweaks
- Provide a default for the haskellPackage option
- Set text values for defaults
- Move hoogleEnv to the top-level & simplify it