We've been using a generic `osbuild-run`, which sets up the build
environment (and works around bugs) for all build roots. It is already
getting unwieldy, because it tries to detect the OS for some things it
configures. It's also about to cause problems for RHEL, which doesn't
currently support a python3 shebang without having /etc around.
This patch changes the `build` key in a pipeline to not be a pipeline
itself, but an object with `runner` and `pipeline` keys. `pipeline` is
the build pipeline, as before. `runner` is the name of the runner to
use. Runners are programs in the `runners` subdirectory.
Three runners are included in this patch. They're copies of osbuild-run
for now (except some additions for rhel82). The idea is that each of
them only contains the minimal setup code necessary for an OS, and that
we can review what's needed when updating a build root.
Also modify the `--build-pipeline` command line switch to accept such a
build object (instead of a pipeline) and rename it accordingly, to
`--build-env`.
Correspondingly, `OSBUILD_TEST_BUILD_PIPELINE` → `OSBUILD_TEST_BUILD_ENV`.
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