Live upgrade and integration tests

It's critical that self-updates do not ever break. Right now I'm very unconfident that we can guarantee this.

To rectify this I intend to do several things:

• Modify the deployment scripts to create archives for the release artifacts.
• Create a new crate rustup-live-test that just contains tests
  • Its test scenarios work by setting RUSTUP_DIST_ROOT into the live archives to simulate updates over time
  • The dist root is configurable so that it can be run against dev-static.rlo, static.rlo, or the file system
• Create a new crate rustup-build-archives that rebuilds the archives. Use this to backfill the live archives and for local testing.
• Create two git branches master-live-test, stable-live-test, with new CI configs
  • These just run the new tests. We'll use these to test the dev deployment before the stable deployment.

cc @alexcrichton @aturon @Diggsey

This is what I intend to do next. I think before doing any further production deployments.

Start by reviewing the deployment scripts and how release artifacts are currently produced. Then examine the proposed rustup-live-test and rustup-build-archives crates, including RUSTUP_DIST_ROOT handling and the planned CI configurations. Done means live-update scenarios run against the configured distribution roots and the dev and stable test branches are exercising them.