Fix pixi cache-dir failure and flaky notebook output check

[woutdenolf]

PR summary

• ewoks install with the pixi manager failed on a Windows CI runner with could not determine default cache directory. Pixi resolves its cache dir from PIXI_CACHE_DIR > RATTLER_CACHE_DIR > XDG_CACHE_HOME/pixi > an OS default that needs LOCALAPPDATA/HOME to be set. PixiManager now falls back to a tempfile.gettempdir()-based PIXI_CACHE_DIR when neither override is already set, so it no longer depends on that OS resolution succeeding.
• test_notebooks[running_workflows.ipynb] flaked on the same runner: a single print(f" {name} =", value) call can be split across more than one Jupyter stream-output record (observed on Windows), so matching the expected substring against one chunk at a time missed it. The check now matches against the concatenation of all stream outputs.

Found via a failing nightly external CI job: https://gitlab.esrf.fr/workflow/external-ci/ewoks-kit/ewoks/-/jobs/2941634

AI Disclosure

• AI tool Claude used for investigation and implementation

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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