Fetch release assets from releases.astral.sh first #18503

This is a tracking issue for using the Astral mirror as the primary source for release artifacts and metadata, before falling back to GitHub Releases.

Things that need to fetch from releases.astral.sh:

• uv installer scripts (uv itself)
• Python installers (python-build-standalone artifacts)
• fetch-download-metadata.py (python-build-standalone release metadata)
• uv format (ruff artifacts)
• uv format (ruff release metadata)
• uv self update (uv itself)
• uv self update (uv release metadata)
• setup-uv (uv artifacts)
• the installer scripts at https://astral.sh/uv/install.sh and https://astral.sh/uv/install.ps1
• check how vscode fetches uv?