Use Pixi for development

[mattwthompson]

Is your feature request related to a problem?

• Environment solution is very slow - only about 10% of workflow runtime is actually spent running tests. Much of this time is spend solving conda environments, some is spent downloading dependencies. Using a lock file skips both of these steps.
• There are many different workflows built off of several different conda environment files and each of them has several combinations of dependencies when tests are actually run. Declaring the test dependencies in one file makes this more clear and feasible to reason through.

Describe the solution you'd like

Use Pixi.

Describe alternatives you've considered

Additional context