text: Title: Replace `freeze.py` with `conda-lock`
  Body: 
  
  ### Proposed change
  
  We use [freeze.py](https://github.com/jupyter/repo2docker/blob/master/repo2docker/buildpacks/conda/freeze.py) to generate fully specified `environment.yml` files out of our `environment.yml` files. It creates a docker image, installs everything, and then lists out the installed packages.
  
  Instead, we could use [conda-lock](https://pypi.org/project/conda-lock/). This resolves `environment.yml` files to fully specified lock files we can use. It doesn't install the packages either, so should be much faster.
  
  ### Alternative options
  
  Do nothing! Our current solution works well.
  
  ### Who would use this feature?
  
  Developers of repo2docker who modify `environment.yml` files. Currently the freezing process can take a while - this would make it faster.
  
  ### How much effort will adding it take?
  
  We need to investigate if conda-lock will actually do what we want. After that, implementing it shouldn't be *too* difficult - a couple hours of work for someone very familiar with the codebase, maybe 8-12h for someone who isn't.
  
  
  ### Who can do this work?
  
  1. Decent python skills
  2. A vague understanding of how conda's environment.yml files work
  