APT: pin intel-microcode/stretch-backports and binary packages built from src:firmware-nonfree/sid to the same level as our custom APT repo.

This allows us to upgrade these packages to a newer version than the one found
in the currently used time-based APT snapshots, by uploading newer packages to
our custom APT repo. But once the version in our time-based APT snapshots of
respectively stretch-backports and sid becomes newer than the one in our custom
APT repo, the former will supersede the latter.