Fix conda-build compatibility regression

[jaimergp]

Description

#449 introduced a regression (identified in conda-forge/rust-feedstock#210) caused by two different bugs in conda.model.channel.Channel: conda/conda#14259 and conda/conda#14258.

It's a kind of a perfect storm situation:

• Requires multi-output recipe with first output being noarch and second output being OS-specific
• Deduplication is based on __eq__ which in Channel deliberately ignores the subdir.
• We were passing subdirs explicitly but this is silently ignored in Channel.urls() if Channel.platform is defined.

The fix is to remove Channel.platform attributes to let the explicit subdirs take over and then we can deduplicate safely.

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• Add / update outdated documentation?