nixos/systemd: Don't use apply for $PATH

When not using apply, other modules can use $PATH as a list instead of
getting a colon-separated list to each /bin directory.

nixos/modules/system/boot/systemd.nix

@@ -257,7 +257,7 @@ let
               pkgs.gnused
               systemd
             ];
-          environment.PATH = config.path;
+          environment.PATH = "${makeBinPath config.path}:${makeSearchPathOutput "bin" "sbin" config.path}";
         }
         (mkIf (config.preStart != "")
           { serviceConfig.ExecStartPre =