Instead of having a static repository checksum, set it dynamically from
the metadata that osbuild-composer last saw. This is implemented in
dnf-json, which returns the checksums for each repository on every call.
This enables the use of repositories that change over time, such as
fedora-updates. Note that the osbuild pipeline will break when such a
repository changes. This is intentional: pipelines have to be
reproducible.
Make each command accept a `repos` key containing repository
descriptions.
Make weldr API pass the repository like this. Nothing should change,
because the repos were the same (Fedora 30).
When osbuild-composer is run as systemd service, we don't want to write
anything into working directory. Currently, we write dnf cache into it.
Instead, let's just use the default dnf cache directory.
