Daemons do not have an /run/user/* dir created

I'm working on the Mir-Kiosk snap. It has Mir running as a "daemon: simple". Mir wants an XDG_RUNTIME_DIR directory for its sockets.

Snapd does set the $XDG_RUNTIME_DIR environment variable for it (/run/user/0/snap.mir-kiosk), but I'm not able to create that directory as /run/user/0 has not been created by snapd.

(I'm creating it myself to work around lp:1656340)

Use this snapcraft file for evidence of this bug:

name: daemon-no-xdgruntimedir
version: "0.1"
summary: Daemon has no XDG_RUNTIME_DIR
description: Daemon has no XDG_RUNTIME_DIR
grade: devel
confinement: strict

apps:
  xdg:
    command: ls -l /run/user/`id -u`
    daemon: simple

parts:
  empty:
    plugin: nil

You'll see a line like this in the journal:
daemon-no-xdgruntimedir.xdg[4702]: ls: cannot access '/run/user/0': No such file or directory