stardoc diff_test fails in downstream rules with FINDSTR error

The stardoc diff_test fails in downstream rules with this error:

```
INFO: From Testing //doc/bzllib:rules.md_difftest:
==================== Test output for //doc/bzllib:rules.md_difftest:
FINDSTR: /= ignored
FINDSTR: /" ignored
FINDSTR: /= ignored
FINDSTR: /" ignored
FAIL: files "doc/bzllib/rules.md" and "doc/bzllib/rules.md_" differ.
```

Steps to reproduce
```
git clone bazel-starlib
cd bazel-starlib
bazel --windows_enable_symlinks test //doc/bzllib:rules.md_difftest --enable_runfiles
```

The issue is that the manifest file cannot be found in diff_test.bzl

Start with diff_test.bzl and reproduce the failure using the provided Bazel command on Windows. Trace why the manifest cannot be found and verify that //doc/bzllib:rules.md_difftest completes without FINDSTR errors and the generated files compare successfully.

Written by the indexing model from the issue text.