feat: support channel_sources from variant configs

[mgorny]

Besides the code being ugly, what I get now is:

 │ │   Channels: 
 │ │    - file:///var/tmp/conda-bld/
 │ │    - conda-forge/label/llvm_rc
 │ │    - conda-forge
 │ │    - conda-forge

I wonder if we should remove the conda-forge default when we're now reading channels from the variant configs.

Fixes conda-forge/conda-smithy#2226

[mgorny]

@wolfv, could you take the initial look at my approach?

[wolfv]

I am fine with this but it should definitely overwrite any default values, and probably even any CLI values.

[mgorny]

In my opinion, CLI should always take precedence — perhaps even override the values coming from configs. But it's your project, so I'm open to doing whatever you think best.

[wolfv]

Well, at least the default value should be gone. I am also fine with default -> variant -> CLI

[mgorny]

Okay, I've made the channel_sources override both config/default for now. Adjusting it to the way I wanted it would require changing the config logic somewhat, so I'll take a look at it later. Solved it.

Do you have a suggestion on how to test this?