builders: prep for build dir move in nix 2.30

[mweinelt]

build-dir no longer defaults to $TMPDIR

The directory in which temporary build directories are created no longer defaults to TMPDIR or /tmp, to avoid builders making their directories world-accessible. This behavior allowed escaping the build sandbox and can cause build impurities even when not used maliciously. We now default to builds in NIX_STATE_DIR (which is /nix/var/nix/builds in the default configuration).

via https://discourse.nixos.org/t/nix-2-30-0-released/66449

[vcunat]

Perhaps it's worth keeping /tmp on tmpfs in addition to this? (though it surely doesn't need to be large)

[h0nIg]

FYI, some of the builds may start failing due to maximum UNIX path length:

       > addr = '/nix/var/nix/builds/nix-build-python3.12-eventlet-0.38.2.drv-108383-1124167291/tmpmeb_8zfzeventlet_test_log_unix_address/socket'
.....
       > >       sock.bind(addr)
       > E       OSError: AF_UNIX path too long

[Mic92]

In our nixos infra we could have our own custom build-dir mounted set to /tmp2 if we wanted. But a better upstream default would be still desired for those that want to mass-rebuild on their own machines.

[mweinelt]

A better upstream default with the same intent could be /nix/tmp, no?