ci: instance type in use has too few resources for existing tests

While working on an improvement for the LXD backed I faced some issues with the CI environment.

Debugging revealed that the provisioned cloud instance has 1 vCPU and 4 GB of memory, which is not a lot.

Running the tests locally with spread from main branch in such a constrained environment gives error prone and prolonged execution locally too.

Some tests execute multiple LXD containers in parallel which obviously will lead to contention between host operating system, test executor (spread), and container/vm workloads. I've also ran out of disk space when adding a couple of system variations to the lxd test case.

I'd suggest we make more resources available for the CI, perhaps providing a `plan` of 'e2-highcpu-8' or 'e2-standard-4' with 40 GB storage would be more suitable?

`e2` = "Day-to-day computing at a lower cost"
`highcpu` = "from 1 to 3 GB memory per vCPU; typically, 2 GB memory per vCPU"
`standard` = "from 3 to 7 GB memory per vCPU; typically, 4 GB memory per vCPU"

No file or test is named in the issue. Start by locating the CI cloud-instance provisioning configuration and identify where the current plan and storage size are set. Compare the available e2 plans, then verify that the existing Spread test suite completes reliably with the selected resources.

Written by the indexing model from the issue text.