Fix mypy precheck failures

[benmoss]

Description

The precommit hook is failing in #626 (among other places) with these errors:

conda_lock/invoke_conda.py:197: error: Non-overlapping equality check (left operand type: "Channel", right operand type: "Literal['defaults']")  [comparison-overlap]
conda_lock/conda_solver.py:349: error: Non-overlapping equality check (left operand type: "Channel", right operand type: "Literal['defaults']")  [comparison-overlap]

I'm not a Python expert but I think the problem is we're comparing a Channel with a string, and equality isn't implemented for those two types. We could either just compare the url field or turn the "defaults" string into a channel, not sure which is preferred.

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[maresb]

Thanks a lot @benmoss for taking initiative on this!!! (I had a very busy week.)

The msys2 stuff predates my involvement in conda-lock, so I needed to do a bit of research to understand what it's all about. I've added a test case to ensure that the newly-activated code is actually correct.

[maresb]

We should be able to cut a new release as soon as everything is confirmed green.