github.py: Get archive from selected version instead of repo HEAD
github.py: Get archive from selected version instead of repo HEAD

Instead of cloning the full repo, and archived copy is downloaded
to save bandwidth. Users who still want the full repo are adviced
to create a local .core without a provider section and clone the
repo themselves for now.

lowRISC/fusesoc,lowRISC/fusesoc,olofk/fusesoc,olofk/fusesoc