Now that the BuildRoot is capable of capturing the output of the
runner and modules (stages, assemblers), there is no need for
using `api.setup_stdio`. Therefore, drop it from all runners and
replace `api.output` with `BuildRoot.output`, which will contain
the output if `api.setup_stdio` is not called from the runners.
Now with `os-release` using `linux` as default ID+VERSION string, we
have a proper fallback name for our blank runner. Rename the blank
runner to `org.osbuild.linux`. It now serves as default fallback for
anything not further specified.