Set `NIX_PATH` in `shellHook` of `shell.nix`.

If `nix-shell` is run interactively on NixOS, in way that has loaded
`/etc/profile` (for example `docker exec <container> nix-shell`), then
`/etc/profile` will be sourced in the shell that nix-shell starts. That
overwrites the `NIX_PATH` set by `shell.nix`.

If we instead set `NIX_PATH` in the the `shellHook`, that always takes
precedence.