v6.4.0

CHANGES:

• Permute the order of Yum groupinstall/install to workaround
  a build issue in CentOS 8 under OverlayFS/Docker. (@avsm)
• Do not install yum-ovl-plugin workaround on CentOS 8. (@avsm)
• Add Fedora 31 and Ubuntu 19.10 (@XVilka @talex5)
• Add Alpine 3.11 and Ubuntu 20.04 (@avsm)
• Remove Ubuntu 19.04 from the supported distro list (@avsm).
• Add a clone_opam_repo optional argument to gen_opam2_distro
  to let the caller decide whether or not to have the git clone
  present in the container (@avsm @talex5).