Avoid copying nixpkgs to the store a second time in nix-shell.

Without this, NIX_PATH points at a copy of nixpkgs in /nix/store.

Specifically, before this patch, NIX_PATH was

/nix/store/rq6cnxd23wzdhvq2a1jj53n5bbrx8yqy-vv70nkylllk1qrvfhl0153r2iy1dfxs9-release2105

and after this patch it is

/nix/store/vv70nkylllk1qrvfhl0153r2iy1dfxs9-release2105

---

This is a small cleanup I noticed while working privatestorageops#382.

shell.nix

@@ -3,8 +3,17 @@ let
 in
 { pkgs ? release2105 }:
 pkgs.mkShell {
+  # When a path (such as `pkgs.path`) is interpolated into a string then nix
+  # first adds that path to the store, and then interpolates the store path
+  # into the string.  We use `builtins.toString` to convert the path to a
+  # string without copying it to the store before interpolating. Either the
+  # path is already in the store (e.g. when `pkgs` is `release2105`) so we
+  # avoid making a second copy with a longer name, or the user passed in local
+  # path (e.g. a checkout of nixpkgs) and we point at it directly, rather than
+  # a snapshot of it.
+  # See https://github.com/NixOS/nix/issues/200 and https://github.com/NixOS/nix/issues/1728
   shellHook = ''
-    export NIX_PATH="nixpkgs=${pkgs.path}";
+    export NIX_PATH="nixpkgs=${builtins.toString pkgs.path}";
   '';
   buildInputs = [
     pkgs.morph