Nix: fix binary cache access

[Riscky]

We have had some binary cache access problems before that seemed impossible to solve.
In the documentation (https://nixos.org/manual/nix/stable/installation/multi-user.html#multi-user-mode) however, we find this:

Multi-user mode has one important limitation: only root and a set of trusted users specified in nix.conf can specify arbitrary binary caches. So while unprivileged users may install packages from arbitrary Nix expressions, they may not get pre-built binaries.

We should add the right users to the trusted-users section and make correct use of the binary caches