Hardcoding a DISPLAY variable is a bad idea because it means that dunst
will only run on X11, and only for that specific configuration.
FWIW, this actually applies to any desktop app run via systemd:
1. `DISPLAY` should be set by `systemctl --user set-environment` elsewhere.
2. `DISPLAY=:0` is not universally valid.
3. This breaks dunst if attempting to run wayland, and requires manually
starting it (eg: not via systemd), or editing the file.
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