Set Up Self-Hosted Kubernetes Runners

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The more model components, models and configurations we support, the more runners that we will need - especially when these runners are running quite intensive, long jobs. Eventually, this could lead to jobs being blocked as there are not enough runners for the organization.

Set up a Kubernetes cluster that utilizes https://github.com/actions/actions-runner-controller on a Nirin instance. In this first instance, no secrets are allowed on jobs that use self-hosted.

See ACCESS-NRI/build-cd#2

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Note, could we get away with having runners that have spack installed initially, rather than using a docker image with spack?
EDIT: It's still useful to have docker containers, in this case.