Release Process: Build and upload release files on CI

[pradyunsg]

Follow up on #253, #265

The aim is to have a workflow where:

• Release manager runs a command locally, which creates the release.
  • Relevant commits + tagging is done on their machine
  • Changes are pushed upstream
  • CI is triggered, running the build-distributions + upload-to-pypi steps.

Right now, all the steps are running locally. This can result in the state of the release manager's machine, affecting the release artifact is built. We're fairly protected, by the defensive nature of the release process in #265, but it's an overall improvement to move the build-distributions step to CI, for introspection at a later date.