fix(interpreter): drop exec_compatible_with from Python interpreter toolchain

[gregmagolan]

Problem

The BCR presubmit fails on Bazel 9 for //cases/uv-deps-650/crossbuild:crossbuild_compile_pyc_test:

While resolving toolchains for target //cases/uv-deps-650/crossbuild:app_bin:
No matching toolchains found for types:
  @@bazel_tools//tools/python:toolchain_type

The toolchain resolution debug log shows why — when evaluating app_bin for an arm64 target on an x86_64 exec host, the arm64 Python interpreter toolchain is rejected with:

Rejected toolchain python_3_12_aarch64_unknown_linux_gnu; mismatching values: aarch64

This is Bazel 9 enforcing exec_compatible_with strictly. The aarch64 Python interpreter was registered with exec_compatible_with = ["@platforms//cpu:aarch64", ...], which doesn't match the x86_64 exec platform. Bazel 8 ignored this mismatch; Bazel 9 enforces it.

Root cause

Since the original python-build-standalone
(PBS) interpreter commit (b2feee5), every Python interpreter toolchain has been registered with:

toolchain(
    name = "python_3_12_aarch64_unknown_linux_gnu",
    exec_compatible_with = ["@platforms//os:linux", "@platforms//cpu:aarch64"],  # WRONG
    target_compatible_with = ["@platforms//os:linux", "@platforms//cpu:aarch64"],
    ...
    toolchain_type = "@bazel_tools//tools/python:toolchain_type",
)

Setting exec_compatible_with on a @bazel_tools//tools/python:toolchain_type toolchain is semantically wrong. The Python interpreter here is a runtime dependency — it ends up inside the virtualenv and runs on the target machine, not on the build host. The exec platform constraint is irrelevant and actively harmful during cross-compilation.

This is distinct from exec_tools_toolchain_type, which represents the interpreter that runs build actions (e.g. compileall in whl_install). That toolchain correctly uses exec_compatible_with — it must run on the exec host, and #903 was specifically designed to handle this case properly. This fix does not touch that path.

Fix

Remove exec_compatible_with from the @bazel_tools//tools/python:toolchain_type registration. target_compatible_with alone is sufficient to select the right interpreter for the target platform.

Test plan

• All e2e tests continue to pass (native builds unaffected — target constraint still selects the right interpreter)
• //cases/uv-deps-650/crossbuild:crossbuild_compile_pyc_test still passes (whl_install exec interpreter still resolved via exec_tools_toolchain_type)

[aspect-workflows]

Bazel 8 (Test)

All tests were cache hits

112 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

Buildkite build #3121 is running...

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Bazel 9 (Test)

e2e

Buildkite build #3121 is running...

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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.