[Dashboard] Fix Path Resolution on Windows

[ijrsvt]

Why are these changes needed?

#39018 makes an implicit assumption that the host OS is Posix and that path resolution follows Posix convention. I don't actually need any of the 'host-specific' aspects of pathlib and only want the Pure Posix path semantics.

Related issue number

Closes #41379

Checks

• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[mattip]

I can confirm that this solves the problem when applied locally on top of 2.8.0

[mattip]

Also closes #41081

[ijrsvt]

Thank you @mattip for checking that this fixes it for Windows! You saved me from having to dust off a Windows machine to test this out 🥳

[ijrsvt]

Merging because test failures are unrelated because they only apply to Docs.