Revert "buildbot-vm.sh: use QEMU bundled with cuttlefish for arm64 as well."

This reverts commit d094263fe569c4201eddecd4ae46b442f828da3a.

Reason for revert: For some reason QEMU for arm64 bundled with
    cuttlefish is very slow compared to the system one -- it causes
    severe timeouts on the LUCI (e.g. Dex2oatImageTest.TestExtension
    was killed because of timeout at 1200000 ms, while with the system
    QEMU it took 589660 ms).

Bug: 283085397

Change-Id: I156309457a70d0e1561e077e5536a7dc8b45812e