hermetic: do not install buildroot via DNF

[praiskup]

Fixes: #1522

[Conan-Kudo]

Why can't we have a fallback?

[praiskup]

Why can't we have a fallback?

C&P from the towncrier snippet: it doesn't make sense because we don't have the bootstrap packages pre-downloaded in the local "offline" repository.

The repository only contains the packages that are needed to re-construct the buildroot repository (no DNF is there, typically).

[Conan-Kudo]

So then... how does a build with dynamic buildrequires work? We need DNF there to be able to install more packages...

[praiskup]

The hook that generates the lock file is "postdeps"; IOW we have all the dynamic build-deps covered in lockfile, and pre-downloaded in the offline repository (but not DNF, as that is typically not in BuildRequires).

[praiskup]

We need DNF there to be able to install more packages...

Yeah and, ... DNF is provided by bootstrap_image_ready=True.

[praiskup]

self-merging, so I can wrap the release, this seems rather trivial anyway