artifact: fix failing tests for sandbox inspection

[tgross]

In #27398 we changed the way we used os.Root to protect the client when merging artifact directories to fix failing tests on Windows where this approach would always fail with "invalid argument". But this has now started failing on unit tests in CI, but only on Linux (where we'll usually have Landlock anyways), specifically on AMD64. While we try to investigate, let's revert #27398 but only on non-Windows.

Because we'll want to make sure we're running the tests on Windows, this changeset also splits out some tests that can't run on Windows because they use OS-specific structs. We'll also tweak a few test assertions to make sure the tests can tolerate being run on either Unix or Windows.

Ref: #27398

Contributor Checklist

• Changelog Entry If this PR changes user-facing behavior, please generate and add a
  changelog entry using the make cl command.
• Testing Please add tests to cover any new functionality or to demonstrate bug fixes and
  ensure regressions will be caught.
• Documentation If the change impacts user-facing functionality such as the CLI, API, UI,
  and job configuration, please update the Nomad product documentation, which is stored in the
  web-unified-docs repo. Refer to the web-unified-docs contributor guide for docs guidelines.
  Please also consider whether the change requires notes within the upgrade
  guide. If you would like help with the docs, tag the nomad-docs team in this PR.

Reviewer Checklist

• Backport Labels Please add the correct backport labels as described by the internal
  backporting document.
• Commit Type Ensure the correct merge method is selected which should be "squash and merge"
  in the majority of situations. The main exceptions are long-lived feature branches or merges where
  history should be preserved.
• Enterprise PRs If this is an enterprise only PR, please add any required changelog entry
  within the public repository.

• If a change needs to be reverted, we will roll out an update to the code within 7 days.

Changes to Security Controls

Are there any changes to security controls (access controls, encryption, logging) in this pull request? If so, explain.

[chrisroberts]

👍