Fix conda packaging (upstream behavior change)

[mara004]

No description provided.

[mara004]

Hmm, despite this and 7dca75c, we are still getting deprecation warnings in workflow runs, see e.g. https://github.com/pypdfium2-team/pypdfium2/actions/runs/12403319048

/usr/share/miniconda/lib/python3.12/site-packages/conda/base/context.py:198: FutureWarning: Adding 'defaults' to channel list implicitly is deprecated and will be removed in 25.3. 

To remove this warning, please choose a default channel explicitly with conda's regular configuration system, e.g. by adding 'defaults' to the list of channels:

  conda config --add channels defaults

For more information see https://docs.conda.io/projects/conda/en/stable/user-guide/configuration/use-condarc.html

The 'defaults' channel might have been added implicitly. If this is intentional, add 'defaults' to the 'channels' list. Otherwise, consider setting 'conda-remove-defaults' to 'true'.