Minimal changes to support windows

[dalito]

Only one test failed on Windows due to a file path comparison that was not written to work cross platform.

This PR:

• fixes the file-comparison to work also on Windows
• adds running on windows-latest to the main CI file.
• activates running tests on merging PRs to main.
• changes the poetry installation to use pipx (as in other linkml-repos)
• Allows to start action runs manually (via workflow_dispatch)

Note that the tests here may fail when the github-api rate limit is hit.

[dalito]

I cannot re-run the failed job here. It failed due to github-api rate limit.
...either I lack permissions in this repo or re-starting runs requires workflow_dispatch which is only added in this PR.