Fix path separator-related bug affecting Windows

[alopezz]

What does this PR do?

It fixes an errors on Windows caused by the use of a Windows path separator (\) where a / is expected. It also enables Windows tests for the downloader on PR's to try to prevent future regressions.

Motivation

Kitchen tests started to fail for windows, presumably due to #13331. E.g. https://gitlab.ddbuild.io/DataDog/datadog-agent/-/jobs/210470215):

            RuntimeError:
       Failed to install integrations package 'datadog-cilium==2.2.1' - Traceback (most recent call last):
         File "C:\Program Files\Datadog\Datadog Agent\embedded3\lib\runpy.py", line 194, in _run_module_as_main
           return _run_code(code, main_globals, None,
         File "C:\Program Files\Datadog\Datadog Agent\embedded3\lib\runpy.py", line 87, in _run_code
           exec(code, run_globals)
         File "C:\Program Files\Datadog\Datadog Agent\embedded3\lib\site-packages\datadog_checks\downloader\__main__.py", line 9, in <module>
           sys.exit(download())
         File "C:\Program Files\Datadog\Datadog Agent\embedded3\lib\site-packages\datadog_checks\downloader\cli.py", line 124, in download
           wheel_abspath = tuf_downloader.download(wheel_relpath)
         File "C:\Program Files\Datadog\Datadog Agent\embedded3\lib\site-packages\datadog_checks\downloader\download.py", line 266, in download
           return self.__download_with_tuf_in_toto(target_relpath)
         File "C:\Program Files\Datadog\Datadog Agent\embedded3\lib\site-packages\datadog_checks\downloader\download.py", line 253, in __download_with_tuf_in_toto
           self.__download_and_verify_in_toto_metadata(target_relpath, target_abspath, target)
         File "C:\Program Files\Datadog\Datadog Agent\embedded3\lib\site-packages\datadog_checks\downloader\download.py", line 229, in __download_and_verify_in_toto_metadata
           root_layout_abspath, root_layout_target = self.__download_in_toto_root_layout()
         File "C:\Program Files\Datadog\Datadog Agent\embedded3\lib\site-packages\datadog_checks\downloader\download.py", line 129, in __download_in_toto_root_layout
           return self.__download_with_tuf(target_relpath)
         File "C:\Program Files\Datadog\Datadog Agent\embedded3\lib\site-packages\datadog_checks\downloader\download.py", line 105, in __download_with_tuf
           raise TargetNotFoundError(f'Target at {target_relpath} not found')
       datadog_checks.downloader.exceptions.TargetNotFoundError: Target at in-toto-metadata\5.core.root.layout not found

It turns out we're not testing on Windows on CI, so this might have been caught before merge, but wasn't.

Additional Notes

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have changelog/ and integration/ labels attached
• If the PR doesn't need to be tested during QA, please add a qa/skip-qa label.

[codecov]

Codecov Report

Merging #13631 (9a0b462) into master (89ae8f4) will increase coverage by 0.00%.
The diff coverage is 90.90%.

Flag	Coverage Δ
datadog_checks_downloader	79.86% <90.90%> (+0.13%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

[alopezz]

Note: this change was necessary so that it uses the right python in Windows, otherwise it would be missing the dependencies.

[iliakur]

afaik this is also what the python docs recommend, so +1 on the change!