This file is owned by root and mode 600 on my system, so the grep is
failing and it's adding a new entry every run.
```sh
-rw------- 1 root wheel 664 Nov 25 15:52 /etc/synthetic.conf
```
Currently if nix-darwin is uninstalled then reinstalled without
rebooting, then the `/run` symlink will still remain and nix-darwin
won't readd `run` to `/etc/synthetic.conf` meaning the system will be
broken on next reboot.
