ci: Increase requested memory size.

ci: Increase requested memory size.

CI builds recently started failing with:

"Memory size for 4.0 vCPU instance should be between 3840MiB and
26624MiB, while 2048MiB is requested."

Ok then, let's ask for 4G instead of 2G.

This may be due to a change in the type of instance used to work around
an outage, per:

https://twitter.com/cirrus_labs/status/1572657320093712384