Cert: Use Python3.8 and bluetooth_packets_python3

* bluetooth_packets_python3 is compiled against Python3.8
  sources in Android tree. Thus cert tests have to use
  Python3.8 if it wants to use compiled bluetooth_packets_python3.so
  from Soong

Bug: 143374372
Test: make and run cert/setup_up_acts.sh and cert/run_cert.sh
Change-Id: I7eabc51999b90229a2f60b62bef5261e6bfe01d9