Pin which version of rust client to use when building the python package

The current python package build process just takes the latest rust binary we have under git. This is not ideal; we may want to select which specific version of the rust binary to pick for a given python package. For example, we may not want to use the latest `v-*-rc.*` rust binary, but we may want to pick a stable release.

We could implement this by improving the python build package script to do a clean checkout of a specific rust binary (rust releases should be tagged with `rust-*`, we could use those tags as reference points).

Regardless, I think it's important to decouple whatever rust's code "lives in main" and what we add to the python package.

@ia0, thoughts?