Describe the bug

Remote building from Nix 2.11.0 on a Mac to an aarch64 Linux machine running 2.8.1, where the remote builder has auto-optimise-store = true, occasionally we're seeing errors on the remote:

error: cannot link '/nix/store/.tmp-link-13949-1705072255' to '/nix/store/.links/09vd...mxd': File exists

this ends up failing the "substitution" and requires retrying. After trying many times, it eventually succeeds.

Expected behavior

1. Optimising the store should not fail if racing to create the link fails.
2. A remote build shouldn't fail if optimising fails.

nix-env --version output

[root@nix-builder:~]# nix-env --version
nix-env (Nix) 2.8.1

$ nix-env --version
nix-env (Nix) 2.11.0

Additional context

I don't believe any changes have been made to Nix since 2.8.1 that impacted auto-optimisation.