if a build directory is accessible to other users it is possible to
smuggle data in and out of build directories. usually this ins only
a build purity problem, but in combination with other issues it can
be used to break out of a build sandbox. to prevent this we default
to using a subdirectory of nixStateDir (which is more restrictive).
Fixes CVE-2025-52991.
Change-Id: Iacfc9b50534de158618c815f9fb99d7dae1be4d0
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.
