Builds failing due to mambaforge deprecation

[brian-rose]

Currently all cookbook builds are failing during the Setup Mambaforge step, e.g. here.

This appears to be due the planned sunsetting of Mambaforge, which is now functionally equivalent to Miniforge. According to this page, it's a "scheduled brownout" to encourage users to switch to Miniforge.

So, let's switch to Miniforge!