CI: use non CPU limited runners with KVM support

[matttbe]

The recent switch to GH Actions reduces the usage of Cirrus CI -- still needed for the tests with a debug kconfig -- but it duplicates CI related code. Also, tests executed on GH Actions still look unstable, probably due to the slow environment (2 CPU without KVM support).

A way to improve the stability and to reduce the maintenance would be to deploy self-hosted runners on machines supporting KVM.

https://docs.github.com/en/actions/hosting-your-own-runners/

[matttbe]

End of last week, I was looking at adding a self-hosted runner. I managed to successfully executed the tests on a self-hosted runner, but KVM was still not used, because the docker image is not executed with enough permissions (--privileged, or --cap-add + mount).

I knew that it was possible to have KVM support, so I tried to find a way to use it with our "Docker container actions", like they do in reactivecircus/android-emulator-runner. Then I found out we can now use KVM support directly with the default GitHub runners.

So I:

• enabled KVM support with GH Actions (with a "workaround"): 677b5ec
• added the 'debug' mode with GH Actions: 6c0b56e
• removed Cirrus-CI support: cc356e6
• (and other clean-ups while at it: https://github.com/multipath-tcp/mptcp_net-next/commits/t/DO-NOT-MERGE-mptcp-add-CI-support/.github/workflows)

So we can close this ticket.