Some kernels disable "unpriveleged user namespaces". This is
unfortunate, but we can still use mount namespaces. Anyway, since each
builder has its own nixbld user, we already have most of the benefits
of user namespaces.

When sandbox-fallback = true (the default), the Nix builder will fall
back to disabled sandbox mode when the kernel doesn’t allow users to
set it up. This prevents hard errors from occuring in tricky places,
especially the initial installer. To restore the previous behavior,
users can set:

  sandbox-fallback = false

in their /etc/nix/nix.conf configuration.

startProcess does not appear to send the exit code to the helper
correctly. Not sure why this is, but it is probably safe to just
fallback on all sandbox errors.