Conda env creation needs to handle all non-default channels

[acwooding]

Right now the script invokes conda-forge even when conda-forge isn't invoked in the environment.yml. We need to be able to pull from other self-hosted channels as well.

Here's a possible spec.

1. Add a section to the environment.yml called channel-order
2. Use scripts_split.py to generate a .yml environment file for each channel (including pip) as well as a channel order file
3. Update the make $(LOCKFILE) step to conda env create each environment file in channel order

[acwooding]

Completed in #243