Set strict channel priority

[kenodegard]

Description

For some reason our test environments were installing the following:

pkgs/main::conda-libmamba-solver
conda-forge::libmambapy
conda-forge::libmamba
...

Mixing defaults and conda-forge like this results in ABI conflicts and renders the libmamba solver unusable (see https://github.com/conda/conda-build/actions/runs/10602558349/job/29384860323). To correct this we set the channel priority to strict.

Closes #5468 Closes #5470

Checklist - did you ...

• [ ] ~Add a file to the news directory (using the template) for the next release's release notes?~
• [ ] ~Add / update necessary tests?~
• [ ] ~Add / update outdated documentation?~