chore(users/Profpatsch): move rust exec-helpers to own subdir

This is so we can use the rust language server for the file.

Change-Id: I8a2fe15ea67fd0e26814fda57bf0cace0d264cae
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7792
Autosubmit: Profpatsch <mail@profpatsch.de>
Reviewed-by: Profpatsch <mail@profpatsch.de>
Tested-by: BuildkiteCI

users/Profpatsch/execline/default.nix

@@ -1,12 +1,6 @@
 { depot, pkgs, lib, ... }:
 
 let
-  exec-helpers = depot.nix.writers.rustSimpleLib
-    {
-      name = "exec-helpers";
-    }
-    (builtins.readFile ./exec_helpers.rs);
-
   exec-helpers-hs = pkgs.haskellPackages.mkDerivation {
     pname = "exec-helpers";
     version = "0.1.0";
@@ -48,7 +42,6 @@ let
 in
 depot.nix.readTree.drvTargets {
   inherit
-    exec-helpers
     exec-helpers-hs
     print-one-env
     ;