Update xmake.lua

packages/s/shaderc/xmake.lua

@@ -19,9 +19,6 @@ package("shaderc")
         if package:config("binaryonly") then
             package:set("kind", "binary")
         end
-    end)
-
-    on_load(function (package)
         if package:config("shared") then
             package:add("links", "shaderc_shared")
         else
@@ -29,6 +26,12 @@ package("shaderc")
         end
     end)
 
+    on_fetch(function (package, opt)
+        if opt.system and package:config("binaryonly") then
+            return package:find_tool("glslc")
+        end
+    end)
+
     on_install("linux", "windows", "macosx", function (package)
         os.execv("python3", {"./utils/git-sync-deps"})
         package:addenv("PATH", "bin")