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

On Windows, if we run `./x.py run src/tools/expand-yaml-anchors/`, the script changes this

https://github.com/rust-lang/rust/blob/11a44adc6fadd84fc11ed986179504e883b528d1/.github/workflows/ci.yml#L8

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.