Updating buildstream is non-trivial with bst-here

Currently, upgrading buildstream to the latest version when using bst-here requires the user to run sudo docker pull buildstream/fedora:latest - while not impossible, not having to know about the image and not having to know about docker is more or less the point of this script.

For now, I added a MR that adds a '-u' flag, but perhaps we should automatically upgrade whenever the script is run.