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

[Gankra]

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.