Cirrus: Use VM instead of Docker for functional tests

[JeremyRand]

Electrum seems to encounter bizarre network connectivity errors (in the form of failed syscalls) when run in a Docker container on Cirrus. This was causing the functional tests on Cirrus to fail, presumably because some of Electrum's Lightning-related socket syscalls were failing. Switching that task to run in a VM instead of a Docker container fixes the failure.

I am not sure whether this is a Cirrus-specific issue or if Electrum has these issues in Docker in general. There is also some nondeterministic behavior in the functional tests that can cause them to sometimes hang; that is out of scope for this PR, but the easy workaround is to restart the affected Cirrus task.

I also fixed the ElectrumX version at v1.15.0, since Electrum master is using the BSV ElectrumX repo (which obviously won't work), and the latest spesmilo/electrumx release has broken wheels.

Fixes #7593

[JeremyRand]

There is also some nondeterministic behavior in the functional tests that can cause them to sometimes hang; that is out of scope for this PR, but the easy workaround is to restart the affected Cirrus task.

For the record, the functional tests task in this PR required 1 restart due to this.

[SomberNight]

Thank you.