Move variant setup from `cosa init` to `cosa build`

Feature Request

Currently, the variant to build is specified at cosa init time and then stored in src/config.json and then read by all other cosa commands: https://github.com/coreos/coreos-assembler/blob/main/src/cmdlib.sh#L177

The variant is also stored in the meta.json after a cosa build but this is apparently not working right now for an uknown reason: https://github.com/coreos/coreos-assembler/blob/main/src/cmd-build#L467

As suggested by @jlebon, we could remove the duplication here by passing the variant as an argument to cosa build only instead and have all later steps & kola read the variant value from the meta.json value.

See:

• https://github.com/coreos/fedora-coreos-pipeline/pull/787
• https://github.com/coreos/coreos-assembler/pull/3274
• https://github.com/coreos/coreos-assembler/pull/2934

We'll also have to change RHCOS CI to adapt to it: https://github.com/openshift/os/blob/master/ci/prow-entrypoint.sh
As well as the pipeline: https://github.com/coreos/fedora-coreos-pipeline

Proposed UX:

cosa init
cosa fetch --variant="rhel-9.0"
cosa build --variant="rhel-9.0"

Start with src/cmdlib.sh and src/cmd-build, tracing how the variant is stored in src/config.json and meta.json. Review the linked coreos-assembler and pipeline changes, plus RHCOS CI at ci/prow-entrypoint.sh. Done means the proposed init, fetch, and build workflow works and later commands and kola use the build metadata without the old init-time setting.