fix(runfiles): assume main repository on Windows (#3578)

With any Python version on Windows, `RUNFILES_*` environment variables
still point to an intermediate stage path instead of the module path,
causing `CurrentRepository()` to raise a `ValueError` since
`rules_python` v1.8.0:
```
ValueError: C:\Users\bot\AppData\Local\Temp\Bazel.runfiles_gh34ij5_\runfiles\+_repo_rules+cpython\my_script.py does not lie under the runfiles root C:/cache/ab1cdef2/execroot/_main/bazel-out/x64_windows-fastbuild/bin/external/+_repo_rules+cpython/install.exe.runfiles
```

This was not the case with `rules_python` v1.7.0. The issue stems from
behavior.

Since #3086 came up with a corresponding workaround in
`//tests/runtime_env_toolchain:toolchain_runs_test`, the proposed fix
simply consists in moving it to `CurrentRepository()`, thus adding
another case to the workaround introduced by #1634 for Python < 3.11. It
therefore leads to assuming the main module path on Windows as well.

Removing the workaround from `CurrentRepository()` would make the test
fail as follows:
```
==================== Test output for //tests/runtime_env_toolchain:toolchain_runs_test:
E
======================================================================
ERROR: test_ran (__main__.RunTest.test_ran)
----------------------------------------------------------------------
Traceback (most recent call last):
[...]
ValueError: C:\Users\user\AppData\Local\Temp\Bazel.runfiles_1f08smy6\runfiles\_main\tests\runtime_env_toolchain\toolchain_runs_test.py does not lie under the runfiles root c:\users\user\_bazel_user\cxxeswjo\execroot\_main\bazel-out\x64_windows-fastbuild-st-c530e4918e48\bin\tests\runtime_env_toolchain\toolchain_runs_test.exe.runfiles
```

Fixes #3579.

(cherry picked from commit f78add7f273bc2fe54ce9187da44abd68588961a)