Move nix folder alongside vagrant (#14132)

shell.nix

@@ -1,7 +1,7 @@
 { avr ? true, arm ? true, teensy ? true }:
 let
   # We specify sources via Niv: use "niv update nixpkgs" to update nixpkgs, for example.
-  sources = import ./nix/sources.nix { };
+  sources = import ./util/nix/sources.nix { };
   pkgs = import sources.nixpkgs { };
 
   poetry2nix = pkgs.callPackage (import sources.poetry2nix) { };
@@ -11,7 +11,7 @@ let
   # --lock" etc. in the nix folder to adjust the contents of those
   # files if the requirements*.txt files change
   pythonEnv = poetry2nix.mkPoetryEnv {
-    projectDir = ./nix;
+    projectDir = ./util/nix;
   };
 in