Remove defaults channel from conda env for docs

As per <https://docs.readthedocs.io/en/stable/guides/conda.html>

Remove defaults channel from conda env for docs
87fb6262 · Ben Mares · d3841a0e · 87fb6262
--- a/doc/environment.yml
+++ b/doc/environment.yml
 name: aesara-docs
 channels:
-  - defaults
  - conda-forge
+  - nodefaults
 dependencies:
  - python=3.7
  - gcc_linux-64