This PR tries fetching the uv artifact from `releases.astral.sh` by
default, only in cases where the artifact would otherwise have come from
`https://github.com/astral-sh/uv/releases/download/`. The checksums are
supposed to be the same for the mirror, and can still come from
`raw.githubusercontent.com/astral-sh/versions`. If the download fails,
we fall back to the original URL.
This avoids hitting GitHub's Releases API which is prone to rate
limiting. As far as I can tell, together with
https://github.com/astral-sh/setup-uv/pull/802 this PR makes a github
token entirely unnecessary for this action.
Towards https://github.com/astral-sh/uv/issues/18503.
