test: drop hardcoded pytest --basetemp so parallel pixi run test in…

[hunger]

…vocations don't race

xdist.setup_node() calls basetemp.mkdir(mode=0o700) without exist_ok=True. With --basetemp=pytest-temp hardcoded in pytest.ini, the second of two concurrent pixi-test invocations panicked with
FileExistsError: '/var/home/tobias/src/pixi/pytest-temp'
Removing the override falls back to pytest's default /tmp/pytest-of-USER/pytest-N/, which auto-uniquifies per invocation.

How Has This Been Tested?

The unit tests

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• This PR contains AI-generated content.
  • I have tested any AI-generated content in my PR.
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Tools: Claude

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added sufficient tests to cover my changes.
• I have verified that changes that would impact the JSON schema have been made in schema/model.py.

[baszalmstra]

I think we did this to ensure the tempdir is on the same drive as the working directory. This enables reflinking which speeds up the tests. You can see that the test duration regressed quite a bit with this PR.

[hunger]

@baszalmstra: Good catch!

The latest and greatest version appends the PID if the basetemp is unset, that seems to work just as well.