feat(uv): let builds declare their execution fleet's platform identity for PEP 517 frontends

[xangcastle]

Context

pep517_frontend (#1483) resets platform_libc/platform_version inside the frontend's exec configuration so its Python build dependencies resolve for the platform it executes on. The reset values default to the host repository rule's probe (CURRENT_PLATFORM_*) — exact whenever the execution platform is the host (every local build), an approximation under remote execution with workers unlike the client. No analysis-time source of truth exists for a foreign worker's libc or version: that is why these are flags rather than constraints.

The wrapper already carries the escape hatch — libc and platform_version attributes that the reset transition reads — but since #1484 wires the wrapper into generated sdist repositories, nothing lets a user set those attributes: the generated pep517_frontend(name = "frontend", actual = ":build_tool") target hardcodes the defaults.

Raised by review on #1484 (repository.bzl thread: "generated repositories provide no way to set the wrapper's libc or platform_version escape-hatch attributes").

Proposal

Thread execution-fleet platform identity into the generated wrapper target. Two candidate shapes:

1. Hub-level setting (preferred?): the execution fleet is a property of the build, not of any one package — e.g. uv.configure(exec_platform_libc = ..., exec_platform_version = ...) flowing into every generated sdist_build repo's frontend target.
2. Per-package via uv.override_package(...), consistent with how other sdist knobs (patches, toolchains, env) are plumbed — more mechanism than the problem needs, but zero new surface.

Either way the default stays the host probe, so local builds are untouched.

Acceptance

• A workspace whose remote workers differ from the client (e.g. macOS client, linux/glibc fleet) can declare the fleet's libc/platform_version once and have every generated frontend resolve its build deps for the worker.
• Analysis test pinning that the declared values reach the frontend's configuration (the flag_probe harness from test(uv): frontend exec configuration must not inherit the target's platform flags #1482 already measures this).
• Docs note replacing the current "remote workers unlike the client are unsupported for sdist builds" caveat with the new knob.