Support multiple conda environments in a single Workflow

[nh13]

Currently, the if one would like multiple conda environments available to the workflow, you can build them into the main docker image (e.g. with conda-env-builder, then activate them as part of each subprocess call. It would be nicer if you could decorate tasks such that a named conda environment could be activated, or a path to a specific environment.yml could be given such that latch would pre-build the environment then activate it (bonus points for knowing when to rebuild if the yml changes). Both of these would improve code readability quite a bit as well as support easier reproducible workflows.