Let gdextension_dir function as only argument

(cherry picked from commit 7a5cbcac21a96fcdd1a7cea0e4f81bca99d4df7a)

tools/godotcpp.py

@@ -273,8 +273,8 @@ def generate(env):
 
 
 def _godot_cpp(env):
-    api_file = normalize_path(env.get("custom_api_file", env.File("gdextension/extension_api.json").abspath), env)
     extension_dir = normalize_path(env.get("gdextension_dir", env.Dir("gdextension").abspath), env)
+    api_file = normalize_path(env.get("custom_api_file", env.File(extension_dir + "/extension_api.json").abspath), env)
     bindings = env.GodotCPPBindings(
         env.Dir("."),
         [