Rearrange selfhosted VM scheduling.

Rearrange selfhosted VM scheduling.

Instead of trying to infer the type of the self hosted tests in each of
the driver scripts (inconsistently...), set one of the following
variables to "true" in the workflow:

VM: tests run in a virtual machine.
EPHEMERAL: tests run on an ephemeral virtual machine.
PERSISTENT: tests run on a persistent virtual machine
REMOTE: tests run on a physical remote host.

EPHEMERAL VMs can have multiple instances of any given VM can exist
simultaneously and are run by a runner pool.  The other types have a
dedicated runner instance and can only run a single test at a time.

Other settings:
SSHFS: We need to sshfs mount over the repo so the workflow can collect
	build artifacts.  This also implies the tests must be run over ssh.
DEBUG_ACTIONS: enable "set -x" in scripts for debugging.