fix coc-flutter on linux

nvim/lua/darcula/setup/100_coc.lua

@@ -142,6 +142,13 @@ coc_config("suggest.noselect", true)
 coc_config("suggest.virtualText", NV_DARCULA_ENABLE_VIRTUAL_TEXT)
 coc_config("sumneko-lua.enableNvimLuaDev", true)
 
+if not utils_os.is_windows then
+  local flutter_sdk = vim.fs.normalize(NV_DARCULA_FLUTTER_SDK_LOCATION)
+  if vim.fn.isdirectory(flutter_sdk) == 1 then
+    coc_config("flutter.sdk.path", flutter_sdk)
+  end
+end
+
 coc_config(
   "languageserver",
   {