add bin/init to s6-rc module

... which suggests it could be better named; init is from
s6-linux-init not s6-rc

modules/s6-rc.nix

@@ -4,6 +4,7 @@ let
     services = builtins.attrValues config.services;
   };
   inherit (pkgs.pseudofile) dir symlink;
+  inherit (pkgs) s6-init-bin;
 in {
   config = {
     environment = dir {
@@ -12,6 +13,9 @@ in {
           compiled = symlink "${s6-rc-db}/compiled";
         };
       };
+      bin = dir {
+        init = symlink "${s6-init-bin}/bin/init";
+      };
     };
   };
 }