horus: Fix key used to lock concurrent accesses to the function cache

[Why]
Using the given `fun()` directly seems ineffective. I think this is
because the function can take variables from the environment; for
example:

    Var = value,
    fun() ->
        do_something(Var)
    end.

[How]
Let's use the same helper that is used to build the function cache key.
This is logical as this lock protects this cache as part of its intent.

Author: dumbbell

src/horus.erl

@@ -416,7 +416,8 @@ to_standalone_fun1(Fun, Options) ->
 to_standalone_fun2(Fun, State) ->
     case to_cached_standalone_fun(Fun, State) of
         undefined ->
-            Lock = {{horus, Fun}, self()},
+            Key = standalone_fun_cache_key(State),
+            Lock = {Key, self()},
             global:set_lock(Lock, [node()]),
             try
                 case to_cached_standalone_fun(Fun, State) of