`startAt = ""` as in `startAt = optionalString false ...` results
in an invalid timer unit (due to "" being promoted to a singleton
list and not filtered out).
Ref: c9941c4b5e
This reverts commit ab6c8643d4. Issue
https://github.com/NixOS/nix/issues/609 has been resolved, the new Nix
version is available after 86eaeb4c0a, and
using nix-collect-garbage has the advantage that the '-d' flag is
available, which nix-store doesn't have.
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.
