Switch to micromamba for CI

[IAlibay]

This is a follow-up from a discussion with @fiona-naughton this morning. It looks like setup-miniconda is just problematic (it fails in odd ways with Python 3.13).

In the core library and increasingly in other places we are switching to using micromamba. It may be better to switch to it?

[lilyminium]

The joys of package managers. Sure, sounds good to me.