Support file:// based repositories in lib.osbuild

[mildas]

In Image Builder test, there might be a need to test a newer oscap version (e.g. from upstream branch) that what is available in compose.

For that, we need to somehow make available custom build packages in compose build. On host, those builds are as file:// repos. That knowledge can be used for getting the builds. Then taking them all and for example creating single big repository and adding the repository among other repos.

[comps]

An osbuild developer confirmed that the composer uses system repositories (/usr/share/osbuild-composer/repositories or /etc/osbuild-composer/repositories) for retrieving oscap, so as long as we include a custom HTTP repository with a newer-than-compose openscap-scanner RPM, it should be used for hardening.