This adds a threat model for `setup-uv` so security scanners can use it
as a baseline in terms of what's in-, and out of scope.
The TM covers credential recipients, executable and cache boundaries,
and release authority. It treats checkout-selected interpreters, paths,
virtual environments, symlinks, and helpers as delegated project
authority unless they override an explicit workflow choice or cross an
independent cache, runner, remote, or publication boundary.
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.
