fix(toolchains): register native_build_toolchain per exec/target platform for sdist builds

[gregmagolan]

Problem

native_build_toolchain (used by pep517_whl for sdist builds) was not registered with @rules_py_tools//:all and it was a single static toolchain with use_target_platform_constraints = True. This only matches when exec == target, but Bazel needs one registered entry per exec platform so it can resolve native_build_toolchain_type for each build host.

Fix

Generate one native_build_toolchain entry per exec platform in toolchains_repo (repo.bzl), so they land in @rules_py_tools//:all which is already registered by all users.

native_build_toolchain_type now resolves automatically on all supported exec platforms via the existing register_toolchains("@rules_py_tools//:all").

[aspect-workflows]

Bazel 8 (Test)

All tests were cache hits

113 tests (100.0%) were fully cached saving 56s.

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Bazel 9 (Test)

All tests were cache hits

112 tests (100.0%) were fully cached saving 60s.

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Bazel 8 (Test)

e2e

All tests were cache hits

52 tests (100.0%) were fully cached saving 40s.

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Bazel 9 (Test)

e2e

All tests were cache hits

52 tests (100.0%) were fully cached saving 60s.

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Bazel 8 (Test)

examples/uv_pip_compile

All tests were cache hits

1 test (100.0%) was fully cached saving 444ms.

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Buildifier

[jbedard]

Can a test be added showing what is being fixed?

[gregmagolan]

Can a test be added showing what is being fixed?

In theory yes, but not without setting up some additional platforms on RBE.

[gregmagolan]

Actually, we if we add an sdist dep to e2e then we'd test that the toolchain is registered properly on at least one platform. I'll update.