ENH: Add defaults to conda channels in build-test-publish GHA

[jhlegarreta]

Add defaults to the list of conda channels when setting up miniconda in the build, test, publish GHA workflow file.

Fixes:

/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

raised for example in:
https://github.com/nipreps/sdcflows/actions/runs/12413644206

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.86%. Comparing base (a50506a) to head (ecb1b33).
Report is 9 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #474   +/-   ##
=======================================
  Coverage   83.86%   83.86%           
=======================================
  Files          30       30           
  Lines        2807     2807           
  Branches      360      360           
=======================================
  Hits         2354     2354           
  Misses        383      383           
  Partials       70       70           

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[jhlegarreta]

The warning is gone:
https://github.com/nipreps/sdcflows/actions/runs/12438444282