Shared nix store across compute nodes

We should configure the compute nodes to directly access the nix store at hut, so we don't have to copy the closures manually when launching SLURM jobs.

A possible option is to configure an overlay file system where we mount the ro nix store by NFS or other remote FS and then set an overlay to always have the kernel to boot:

https://discourse.nixos.org/t/sharing-nix-store-between-containers/9733/16 https://github.com/NixOS/nixpkgs/blob/8346dc04b31bc4ade35bd15a72e6cc40c8ac2f73/nixos/modules/installer/netboot/netboot.nix#L39-L70 https://github.com/NixOS/nixpkgs/blob/8346dc04b31bc4ade35bd15a72e6cc40c8ac2f73/nixos/modules/installer/netboot/netboot.nix#L106-L108