Add tests for micromamba, transmuation and miniforge

[chrisburr]

This is the promised PR to improve the test coverage, in particular:

• Run all of the examples with both conda-standalone and micromamba
• Add an example that looks like miniforge (not technically needed, but tests a anaconda.org hosted channel and helps the next point)
• Adds an example which uses transmutation of packages to .conda

Along the way I found a bug that the CI often tested the latest release from the defaults channel rather than the current commit. I've fixed this by setting channel_priority: strict.

[chrisburr]

I'm not testing on Windows as micromamba isn't used for Windows+miniforge and I don't have the means to debug it easily. (It seems like the tests deadlock in the CI.)

[chrisburr]

For reference here is the deadlocked pipeline for Windows: https://github.com/conda/constructor/runs/5536429721?check_suite_focus=true