Set CONDA_BLD_PATH when using pixi during osx build

[hadim]

Checklist

• Added a news entry
• Regenerated schema JSON if schema altered (python conda_smithy/schema.py)

This is an attempt to fix #2145. You can see it in action at conda-forge/docling-parse-feedstock#3

Note the fix is a simple and naive patch but looks fragile to me. Please let me know if you can think of another solution.

[beckermr]

I wonder if we should set this on all platforms?

Also, the code that finds the outputs is in the ci-setup feedstock. We need to consult that code to ensure what we are doing here works with it.