Use mamba in appveyor
This MR updates the appveyor CI jobs to use mamba.
Maintenance will be performed on git.ligo.org, containers.ligo.org, and docs.ligo.org on Tuesday 24 September 2024 starting at approximately 9am PDT. It is expected to take around 30 minutes and there will be several periods of downtime throughout the maintenance. Please address any comments, concerns, or questions to the helpdesk.
This MR updates the appveyor CI jobs to use mamba.