build: Export runtime_env_toolchain_interpreter.sh file (#3471)

I am working on bazelbuild/bazel#27674 towards
bazelbuild/bazel#10225 to flip
`incompatible_no_implicit_file_export`. I found that
https://github.com/bazelbuild/bazel/blob/master/src/test/py/bazel/bzlmod/bazel_repo_mapping_test.py
and
https://github.com/bazelbuild/bazel/blob/master/src/test/py/bazel/bzlmod/bazel_module_test.py
require `runtime_env_toolchain_interpreter.sh`. Since it is no longer
implicitly exported after flipping the flag, we have to export it
explicitly.

Author: hofbi

python/private/BUILD.bazel

@@ -28,6 +28,8 @@ package(
 
 licenses(["notice"])
 
+exports_files(["runtime_env_toolchain_interpreter.sh"])
+
 filegroup(
     name = "distribution",
     srcs = glob(["**"]) + [