`./x.py run src/tools/expand-yaml-anchors/` is directory separator-sensitive

[JohnTitor]

On Windows, if we run ./x.py run src/tools/expand-yaml-anchors/, the script changes this

rust/.github/workflows/ci.yml

Line 8 in 11a44ad

# src/ci/github-actions/ci.yml

to src/ci/github-actions\ci.yml.

This isn't ideal, though it's quite a minor point. I think it'd be nice if we could make it insensitive.