Improvements to the install script and associated CI steps (leveraging mamba)

This MR:

1. Configures the install.sh script to use mamba over conda if a user has mamba installed. If a user has neither installed, the script installs mamba.
2. Modifies the CI file so that we can just directly use the install.sh script. This has always been a goal (to avoid duplicated code in the CI file) but for some reason I wasn't able to figure it out until now. Combined, these changes leverage mamba to solve the environment in the CI instead of conda. This results in tests that are > 5x faster (and therefore less likely to fail because they're timing out).

Closes #29 (closed).