fix(py_test): fix Windows crash in py_test main validation

[rdesgroppes]

py_test main validation (--validate_test_main=enabled) crashes on Windows, because it invokes the interpreter via exec_tools.exec_interpreter, which resolves through current_interpreter_executable()'s relocated copy of the interpreter.

The crash may be reproduced on existing tests in the repo, for instance:

bazel build \
  --@rules_python//python/config_settings:validate_test_main=enabled  \
  //tests/validate_test_main:validate_test_main_test
...
ERROR: .../tests/validate_test_main/BUILD.bazel:3:8: Validating py_test
 main //tests/validate_test_main:validate_test_main_test failed:
 (Exit -1073741515): python.exe failed: error executing
 PyValidateTestMain command

cd tests/integration/validate_test_main
bazel build \
  --@rules_python//python/config_settings:validate_test_main=enabled \
  //:good_test
...
ERROR: .../tests/integration/validate_test_main/BUILD.bazel:11:8:
 Validating py_test main //:good_test failed: (Exit -1073741515):
 python.exe failed: error executing PyValidateTestMain command

-1073741515 is STATUS_DLL_NOT_FOUND: the relocated copy can't find its DLLs beside itself.

This is also the Windows-local manifestation of #2703 (exec_interpreter broken on RBE): the same relocation severs the interpreter from files resolved relative to itself, just triggered differently: RBE's copy materialization there, a lack of symlink privilege here.

Colocating the DLLs alone (a first attempt) traded this for a second, still fatal error, ModuleNotFoundError: No module named 'encodings', because the copy still can't find its stdlib.
Patching each missing file individually doesn't scale: DLLs today, stdlib tomorrow, whatever else a future toolchain needs beside itself after that.

_maybe_add_test_main_validation now uses actions_run() with exec_runtime instead of exec_tools.exec_interpreter's relocated DefaultInfo.files_to_run, matching PyExecToolsInfo.exec_interpreter's own documented recommendation and the pattern common.bzl's actions_run() and py_zipapp_rule.bzl already use.
exec_runtime.interpreter is the real file, used directly, with its real files as plain action inputs, so nothing is relocated and nothing loses its siblings.

With the proposed fix¹, above examples now build cleanly, with no relocated runfiles tree for the interpreter at all, and tests/integration/validate_test_main's inert_test still fails with its intended "will not run any tests" message rather than a crash.

tests/integration/validate_test_main_test is the corresponding integration test, but it was not exercised on Windows, where it was failing on OSError: [WinError 193] %1 is not a valid Win32 application in tests/integration/runner.py's Bazel-in-Bazel invocation, itself unable to run bazel_from_env's #! shebang line the way POSIX's exec does.
The present change therefore fixes this, by resolving the shebang's interpreter itself, and enables the test on Windows.

Footnotes

1. This does not fix exec_interpreter/exec tools exec_interpreter broken on RBE #2703 itself: precompile.bzl still resolves the interpreter via the relocated path and would need the same migration. ↩

[rickeylev]

Thanks for the clean PR! Just needed a couple style fixes. Otherwise LGTM

[rdesgroppes]

Thanks for the clean PR! Just needed a couple style fixes. Otherwise LGTM

@rickeylev merci !