Move away from treefile-apply #1850
Description
To be able to work around https://gitlab.com/fedora/bootc/tracker/-/issues/59, we currently dnf versionlock all the base packages. It came up recently that rpm-ostree treefile-apply should probably pass --no-best to dnf install to allow dnf to install the older matched versions, even if there are newer packages in the repo.
But it's also annoying to have to land fixes in rpm-ostree for this.
Nothing else currently AFAIK uses treefile-apply. It was useful in that interim state where we had to support both base composes and layered builds, but now that we've cut over, we have much more freedom in how to organize ourselves. I don't think much will happen in treefile-apply. It has very limited scope purposely.
It wouldn't be hard to rework the build to just do the equivalent things that treefile-apply did. We don't have to keep treefiles. E.g. all the postprocess bits could be directly inlined in build-node-image.sh, or scriptlets in a directory, packages can be listed cosa-style, etc... And then we'd be able to trivially do fixes like add --no-best.