Update release generation

The automatically generated Autotools files, which we need in the release branches, require additional license files. The tool we use for the license checking, reuse, does not like it when there are redundant license files in the repo, therefore, we cannot have the required extra licenses to be simply added to the repo. On the other hand, when we run reuse for release branches with the generated Autotools files, reuse complains about missing licenses. As a workaround, !42 (merged) introduced an extra step that runs reuse download --all if the configure script is present in the repo. It was a bad idea because we should deliver the library with the full set of licenses, i.e. the release branches are supposed to have the extra licenses. It also hinders further refactoring of the Check licenses CI job. This PR drops the workaround and updates the release generation script so that the required licenses are fetched and added to the repo.