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