Merge pull request #4560 from gfaster/which-nix-shell

add which to shell.nix to build with --pure

shell.nix

@@ -3,6 +3,7 @@ pkgs.mkShell {
   name = "odin";
   nativeBuildInputs = with pkgs; [
     git
+    which
     clang_17
     llvmPackages_17.llvm
     llvmPackages_17.bintools