x/build/cmd/debugnewvm: investigate adding serial support to EC2 instances

[cagedmantis]

In the process of adding EC2 builders #36841 debugnewvm was updated to support creating instances on EC2. GCE instances support reading from the serial support from VMs. We should investigate whether it is possible and possibly implement this with EC2 instances.

/cc @andybons @dmitshur @toothrot