When running `image-builder` warnings are emitted during manifest
generation. Depending on the definitions or customizations packages
can be excluded from groups which leads to:
```
No match for group package "dracut-config-rescue"
```
Interspersing with normal output. Let's pass along another byte buffer
for the warnings from manifest generation to be written into.
Note that this also needs plumbing [1] to land in `images` first.
[1]: https://github.com/osbuild/images/pull/1384
Signed-off-by: Simon de Vlieger <supakeen@redhat.com>
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