Fetch uv from Astral mirror during self-update

[zsol]

Following #18679, this PR makes uv fetch the installer script from the Astral mirror (similarly to how we fetch the ndjson for version metadata), and then run the script directly, emulating what axoupdater would do.

This completes the uv self update (uv itself) item in #18503

Test Plan

Added tests, plus did this manually:

❯ cargo build --features self-update
[...]
❯ cp target/debug/uv ~/.local/bin/
❯ uv self update 0.11.0 -vv
DEBUG uv 0.11.1 (aarch64-apple-darwin)
DEBUG Using request connect timeout of 10s and read timeout of 30s
info: Checking for updates...
DEBUG Using official public self-update path
DEBUG Using request connect timeout of 10s and read timeout of 30s
TRACE Handling request for https://releases.astral.sh/github/versions/main/v1/uv.ndjson with authentication policy auto
TRACE Request for https://releases.astral.sh/github/versions/main/v1/uv.ndjson is unauthenticated, checking cache
TRACE No credentials in cache for URL https://releases.astral.sh/github/versions/main/v1/uv.ndjson
TRACE Attempting unauthenticated request for https://releases.astral.sh/github/versions/main/v1/uv.ndjson
DEBUG Resolved self-update target to `uv==0.11.0`
DEBUG Using request connect timeout of 10s and read timeout of 30s
TRACE Handling request for https://releases.astral.sh/github/uv/releases/download/0.11.0/uv-installer.sh with authentication policy auto
TRACE Request for https://releases.astral.sh/github/uv/releases/download/0.11.0/uv-installer.sh is unauthenticated, checking cache
TRACE No credentials in cache for URL https://releases.astral.sh/github/uv/releases/download/0.11.0/uv-installer.sh
TRACE Attempting unauthenticated request for https://releases.astral.sh/github/uv/releases/download/0.11.0/uv-installer.sh
success: Downgraded uv from v0.11.1 to v0.11.0! https://github.com/astral-sh/uv/releases/tag/0.11.0

[codspeed-hq]

Merging this PR will not alter performance

✅ 5 untouched benchmarks

---

_{Comparing zsol/jj-llkyowxrwzll (bc4f5ce) with main (944f009)}