random() is not thread-safe, it relies on global state, and calling it
from worker threads can result in multiple threads producing the same
value. It also doesn't guarantee unique values even in single-threaded
use.
Use an atomic counter for the use-case of generating temporary paths,
and switch to a thread-local RNG for the one remaining call.
This will probably fix https://github.com/NixOS/nix/issues/7273 though
I'm not willing to risk corrupting my store to find out.
Change-Id: I4c4c4c9796613573ffefd29cc8efe3d07839facc
