Fix `--nozip_undeclared_test_outputs` on Windows

Without this change, testing with `--nozip_undeclared_test_outputs` always fails with this error:

```
FATAL: MappedOutputFile(): CreateFileW() failed: (error: 3): The system cannot find the path specified.
```

Closes #16973.

PiperOrigin-RevId: 494953454
Change-Id: I8338e897a9e4f4fdcd6dad511ee8e777182fefcd