For easier pipeline integration and finger compatibility, support having `cosa build` use the new container native path to build the `ostree` artifact rather than the previous direct `rpm-ostree compose tree` path.

Instead of extending the hacky and complex cmd-build bash script, I'd say let's instead integrate this at the `coreos-assembler.go` level. The container native path renders a lot of code in `cmd-build` obsolete.

Roughly:

1. Add a new e.g. `cosa buildoci` command. What that command would do is essentially:

1. 1. `cosa supermin-run podman build ...`
   2. `cosa import ...` (see COS-3309)
2. In `coreos-assembler.go`, make `cosa build` actually just call `cosa buildoci` if e.g. `COSA_BUILDOCI=1`.

Having it be env var based makes it easier down the line to integrate into the pipeline since it already supports env vars.