fix build number fetch (again)

i think we might need to fetch every script until we find the one with build number
(i.e. the "main script" is no longer fixed in the html)

src/startup.cpp

@@ -43,8 +43,8 @@ std::optional<Glib::ustring> GetJavascriptFileFromAppPage(const Glib::ustring &c
         start_position += str.size();
     }
 
-    if (matches.size() >= 6) {
-        return matches[matches.size() - 6];
+    if (matches.size() >= 7) {
+        return matches[matches.size() - 7];
     }
 
     return {};