rustdoc: align the doctest runner with the test platform (#4279)

Build the compiled-doctest runner for Bazel's test execution platform
and preserve its executable suffix. Reject cross-built legacy doctests
with a diagnostic explaining that they must use nightly compiled
doctests, since legacy doctests execute the compiler during testing.

The Rustdoc analysis suite passes on macOS in stable-only, nightly-only,
and Bazel 7.4.1 configurations. A current-Bazel platform regression
fails with the old runner transition and passes with path mapping
enabled. Compiled-only tests inherit the selected toolchain and run only
when nightly compiled doctests are enabled. Windows execution remains
for CI.

Closes #4293.