Add RC conda label to publish_conda_package

[hoxbro]

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• issues: resolves [FEATURE] Add a conda channel for rc releases  #13963
• tests added / passed
• release document entry (if new feature or API change)

This makes it so RC and full releases will be published to the bokeh conda channel under the label rc.

I don't have the power to create the rc label, but this needs to be done before this PR is merged.

[mattpap]

I don't have the power to create the rc label, but this needs to be done before this PR is merged.

I don't see any option for creating labels on anaconda.org (I can only assign a new label to existing uploads), nor the documentation says anything about creating labels. I figure a new label will be created when uploading with --label rc for the first time?

[bryevdv]

This was my understanding as well. @philippjfr can you comment?

[philippjfr]

Yes, also my understanding, i.e. adding --label rc should be all that's needed.

[github-actions]

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