nix-daemon.conf.in: add tmpfiles file to create nix/daemon-socket directory

nix-daemon.socket is used to socket-activate nix-daemon.service when
/nix/var/nix/daemon-socket/socket is accessed.

In container usecases, sometimes /nix/var/nix/daemon-socket is
bind-mounted read-only into the container.

In these cases, we want to skip starting nix-daemon.socket.

However, since systemd 250, `ConditionPathIsReadWrite` is also not met
if /nix/var/nix/daemon-socket doesn't exist at all. This means, a
regular NixOS system will skip starting nix-daemon.socket:

> [  237.187747] systemd[1]: Nix Daemon Socket was skipped because of a failed condition check (ConditionPathIsReadWrite=/nix/var/nix/daemon-socket).

To prevent this from happening, ship a tmpfiles file that'll cause the
directory to be created if it doesn't exist already.

In the case of NixOS, we can just add Nix to `systemd.tmpfiles.packages`
and have these files picked up automatically.

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