Handle buildbox-casd dependency more robustly

Currently the gitlab-ci.yml points at a precompiled date stamped tarball of buildbox-casd, i.e https://buildbox-casd-binaries.nyc3.cdn.digitaloceanspaces.com/buildbox-casd-x86_64-linux-20191104-598100dd.tar.xz

Having this hardlinked dependency means that the automated pipeline for the weekly benchmark falls-over when a given change is merged to bst which requires a new casd version (as can be seen here https://gitlab.com/BuildStream/benchmarking/benchmark_debian/-/jobs/346769837). There's also no built-in mechanisim for handling such updates as a bst user, however failure messages point towards the underlying issue.