Fix crossdirect rust test

Currently running test_crossfirect locally may fail if selected device does not exist on stable branch.

Fix it by switching to qemu-amd64 before running this test.

Test plan:

$ pmbbootstrap config device samsung-klte
$ pytest -vv ./test/test_crossdirect.py  # fails without this MR, suceeds with

While at it, switch to more recent stable branch, v20.05 is now unmaintained