fusuma: use lib.makeBinPath for service PATH

The local makeBinPath helper reimplemented lib.makeBinPath with a
non-strict foldl accumulator. Use the stdlib function instead: it is
strict and removes the duplicated logic.

modules/services/fusuma.nix

@@ -57,10 +57,6 @@ let
     '';
   };
 
-  makeBinPath =
-    packages:
-    lib.foldl (a: b: if a == "" then b else "${a}:${b}") "" (map (pkg: "${pkg}/bin") packages);
-
 in
 {
   meta.maintainers = [ lib.maintainers.iosmanthus ];
@@ -127,7 +123,7 @@ in
       };
 
       Service = {
-        Environment = "PATH=${makeBinPath cfg.extraPackages}";
+        Environment = "PATH=${lib.makeBinPath cfg.extraPackages}";
         ExecStart = "${cfg.package}/bin/fusuma";
       };