Fill Python requests with platform information during automatic fetches

[zanieb]

Closes https://github.com/astral-sh/uv/issues/4800

We do this during install — it's an important step to ensure the request has the platform information in it.

[zanieb]

@konstin could you check this resolves the issue on Linux please?

[zanieb]

The need for a lightweight testing story for Python toolchain commands grows...

[konstin]

Still failing for me:

$ rm -rf ~/.local/share/uv/ ~/.cache/uv/ && cargo run -q -- tool install --preview -p 3.12 --force black --python-preference only-managed
error: Failed to query Python interpreter at `/home/konsti/.local/share/uv/python/cpython-3.12.3-macos-aarch64-none/install/bin/python3`
  Caused by: Exec format error (os error 8)

[zanieb]

@konstin that's because I didn't change it in the other place Charlie pointed out

[charliermarsh]

I'm gonna add and merge for now -- we can revisit if we wanna do something better.