systemd: move systemd-tmpfiles-setup-dev.service back to early boot
systemd: move systemd-tmpfiles-setup-dev.service back to early boot

It was originally moved because of nixops autoLuks feature which
has been unsupported for a while.

See:
* https://github.com/NixOS/nixpkgs/issues/62211
* https://github.com/NixOS/nixops/pull/1156#issuecomment-605339705

systemd-tmpfiles-setup-dev.service needs to run very  early (even before
udev runs) because udev rules assume static device nodes already exist
even before udev is started. If these static device nodes do not exist;
systemd might have trouble mounting filesystems that require static
device nodes (like loopfs and btrfs).

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