ci: Migrate tests to GitHub actions

[stefanprodan]

GitHub Actions migration (ref: #2944):

• run unit tests
• verify codegen
• build binaries and containers
• run e2e tests (four Kubernetes Kind clusters matrix)

GH Actions specific changes:

• cache go modules
• run one Kubernetes cluster per VM
• upload fluxd container image, fluxctl, kubectl, kustomize and sops to GH artifacts
• download the artifacts in e2e jobs to avoid rebuilding binaries and containers
• cleanup artifacts daily

Conclusions:

• the random kind/docker/kubeadm failures that we experienced with CircleCI are gone
• no more sudo killall apt-get, apt-get doesn't hang like in CircleCI
• the e2e tests take the same amount of time on GH Actions free as in CircleCI paid account
• the major downside to GH Actions is the lack of docker image caching between jobs
• you have to use docker save/load and upload/download artifacts between jobs
• you can't delete artifacts during a GH workflow run
• GH Actions UI is unreliable and buggy, most of the time the logs are not displayed until the job finishes

Overall I'm happy with GitHub Actions even though it was a painful migration.

[dholbach]

Well done @stefanprodan - you got this done in next to no time!

[hiddeco]

Would something like

        test: ['1/4', '2/4', '3/4', '4/4']

be an option (with additional logic added to run-gh.bash)? This would make (newly) added tests run without (forgetting) to add them here.

[stefanprodan]

I've balanced the tests to take the same amount of time, for example running 15 in shard 2 will take 18m

[hiddeco]

Ack.

[stefanprodan]

I think we should refactor the tests and have the balancing logic in bash, but this can come later.