Use mamba instead of conda for environment

mamba is a much faster, drop-in replacement for conda. This is because its solver is implemented in C instead of Python. By using mamba we can expect local installs to go much faster, and we can also benefit from faster CI (Pipelines) run times. This will make us (the developers / users) happier, and will make new lsforce users — who are installing the code for the first time — happier, too.

To make this happen, we'd need to update the following files:

• install.sh — have it install mamba instead of conda (and/or we can simply have it install using mamba if it detects mamba and conda if it detects conda, since the installation commands are basically identical)
• .gitlab-ci.yml — update to use mamba (currently we're using an image with conda already installed: image: ${DEVOPS_REGISTRY}usgs/conda:latest so this will involve using a mamba image if one exists else requesting that one be created)

Ping @kallstadt & @ecollins