prefer fetching cargo-dist's own artifacts/metadata from axo-releases

There are several situations where cargo-dist needs to fetch itself or axoupdater:

* in ci to bootstrap with curl-sh/irm-iex, we need to fetch the shell script, and then fetch whatever it says to fetch
* when using the sidecar axoupdater binary, we need to fetch that (falling back to building from source)
* when doing a selfupdate, we have to hit a releases api *and* fetch artifacts

This is currently a hodgepodge of fetching from github or axo releases. We should prefer axo-releases wherever possible, as the github cdn has been incredibly flakey and the api is prone to ratelimiting us.

No files or tests are named. Start by locating the fetch paths for CI bootstrap scripts, the sidecar axoupdater binary, self-update metadata, and artifacts, then compare their GitHub and axo-releases sources. Done means cargo-dist prefers axo-releases wherever possible while retaining the stated source-build fallback.

Written by the indexing model from the issue text.