Define {builder,fs}.nix for utils.nix

Re-export everything from utils.nix.

utils/builder.nix

@@ -0,0 +1,11 @@
+{ pkgs, ... }:
+
+let
+  inherit (pkgs) writeShellScriptBin;
+in {
+  # Create a derivation that creates an executable shell script named `as` that
+  # calls the program located at `path`, forwarding all of the arguments.
+  wrapNonNixProgram = { path, as }: writeShellScriptBin as ''
+    exec ${path} "$@"
+  '';
+}