Issue description

Setting both
boot.tmpOnTmpfs
as well as
zramSwap.enable
results in in an ordering cycle:

sysinit.target: Found ordering cycle on systemd-timesyncd.service/start
sysinit.target: Found dependency on tmp.mount/start
sysinit.target: Found dependency on swap.target/start
sysinit.target: Found dependency on dev-zram0.swap/start
sysinit.target: Found dependency on zram-init-zram0.service/start
sysinit.target: Job systemd-timesyncd.service/start deleted to break ordering cycle starting with sysinit.target/start

..and similar output.

Steps to reproduce

Set both zramSwap.enable and boot.tmpOnTmpfs to true.

Technical details

Please run nix-shell -p nix-info --run "nix-info -m" and paste the
results.

• system: "x86_64-linux"
• host os: Linux 4.18.10, NixOS, 19.03pre153673.a70d3bab72d (Koi)
• multi-user?: yes
• sandbox: yes
• version: nix-env (Nix) 2.1.2
• channels(root): "nixos-19.03pre153673.a70d3bab72d"
• nixpkgs: /nix/var/nix/profiles/per-user/root/channels/nixos