- Set default attach_timeout to 40s
- Increase timeout only after failed attach attempts

This modification addresses an issue in the RFSim-5G 2x2 test where the UE
often fails to attach within the initial 20s window. The timeout may cause
the UE to restart, which leads to two UEs appearing in the gNB log. By
increasing the initial timeout to 40s, we avoid unnecessary restarts.