need CI to run in cgroup v2 environment

[kolyshkin]

This repo needs additional CI set up in order to run tests on a cgroup v2 box.

Alas, it's not available from GHA per se, and currently the only distro that enables cgroup v2 by default is Fedora.

Two ways I'm aware of are:

1. Use mac os x instance on GHA CI, start a Vagrant box running Fedora (this is what used by e.g. https://github.com/moby/sys, for different reasons though -- need openat2(2) syscall).
2. Use Cirrus CI -- alas in this case we also have to spawn a Vagrant box running Fedora, since Cirrus CI infra does not provide hosted Fedora. This is what e.g. https://github.com/opencontainers/runc is using.

Either way is fine, though (2) turned out to be slightly faster and more reliable. For things such as unit tests, I guess, GHA is fine, too.