Add a `bst build --no-pull` mode

I've been building CI pipelines like this where we bst build lots of systems. A lot of the time the artifacts are already cached at https://ostree.baserock.org. Each CI job takes time to download every artifact, which isn't really necessary as we just want to test that it builds.

It would be useful if we could say "run a build if necessary, but exit with success if the artifact exists in the remote cache" easily.