Remove defaults channel from conda envs

[charlesbluca]

As pointed out by @jakirkham, both Dask and RAPIDS have moved away from the defaults in CI in favor of conda-forge.

This PR removes the defaults channel from the conda environment files, so that all packages will get picked up from conda-forge.

[codecov-commenter]

Codecov Report

Merging #384 (749511a) into main (7620d6d) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #384   +/-   ##
=======================================
  Coverage   95.60%   95.60%           
=======================================
  Files          67       67           
  Lines        2933     2933           
  Branches      547      547           
=======================================
  Hits         2804     2804           
  Misses         79       79           
  Partials       50       50           

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