explicitly exclude defaults channel

make sure we're always using conda-forge

it's unlikely any packages came from defaults for users (will only happen for packages not on conda-forge, and would often result in a broken env anyway),
but this ensures defaults channel is opt-in.

Author: minrk

repo2docker/buildpacks/conda/install-base-env.bash

@@ -30,7 +30,6 @@ export PATH="${PWD}/bin:$PATH"
 cat <<EOT >> ${CONDA_DIR}/.condarc
 channels:
   - conda-forge
-  - defaults
 auto_update_conda: false
 show_channel_urls: true
 update_dependencies: false