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.

shell.nix

@@ -3,7 +3,9 @@ let
 in
 { pkgs ? release2105 }:
 pkgs.mkShell {
-  NIX_PATH = "nixpkgs=${pkgs.path}";
+  shellHook = ''
+    export NIX_PATH="nixpkgs=${pkgs.path}";
+  '';
   buildInputs = [
     pkgs.morph
     pkgs.vagrant