Fix editor saving to downloadedmaps when loading current map

When loading a map from the `downloadedmaps` folder with the editor file browser it was already correctly ensured that the map cannot be saved again to this folder, but this was not checked when the current map is loaded directly from the `downloadedmaps` folder with the "Load current map" function.

Closes ddnet#9529.

src/game/editor/editor.cpp

@@ -8910,7 +8910,7 @@ void CEditor::LoadCurrentMap()
 {
 	if(Load(m_pClient->GetCurrentMapPath(), IStorage::TYPE_SAVE))
 	{
-		m_ValidSaveFilename = true;
+		m_ValidSaveFilename = !str_startswith(m_pClient->GetCurrentMapPath(), "downloadedmaps/");
 	}
 	else
 	{