s6: use exec in the run trampoline, to forward file descriptors and keep a cleaner process tree

modules/users/s6-rc.nix

@@ -9,10 +9,10 @@ let
         name = "s6-${name}-run";
         destination = "/${normalizeName name}/run";
         executable = true;
-        # text = run;
+        # TODO: consider using `makeWrapper`/`makeWrapperBin`?
         text = ''
           #!/bin/sh
-          ${run}
+          exec ${run}
         '';
       })
     ]