add conda-forge integration

[0xbe7a]

Until anaconda/anaconda-client#696 is merged, anaconda-client cannot be used to upload packages with conda-forge-ci-setup. This PR adds a conda-forge upload as a special case of the anaconda upload, which makes a copy request after the upload.

conda-forge-ci-setup also uses conda-build under the hood, so this allows testing of rattler-build for conda-forge packages without being blocked by conda-build. In the long run, we probably want to integrate rattler-build properly into conda-forge-ci-setup / fix existing problems of conda-build / anaconda-client when uploading packages built by rattler-build

[0xbe7a]

Currently the channel / label is hardcoded (see https://github.com/conda-forge/polarify-feedstock/blob/rattler-build/.scripts/build_steps.sh#L83). Do you think we could we could read this information from the package itself / include it in the package @wolfv ? I'm not quite sure if the "label / channel" concept is anaconda.org or a general conda concept and if it should leak into the package format.

The other two ways I see to give the information to the upload command is to run yq during the build and parse the conda_build_config itself or to do that at render time with conda_smithy and templatize it into the command as an argument

[wolfv]

Isn't the channel / label stuff something that conda-smithy would handle? It's got to be somewhere passed in to the recipe.

Instead of using yg you can also just use the YAML parser of rattler-build. The .ci_config/...yaml files are already evaluated so that they contain "pure YAML" and should be easy to parse (as long as you select the proper one).

At the end of the day, a label is just a channel. Personally not a huge fan of labels, so if we could only talk about channels I would probably prefer it but since this code is anaconda specific anyways, i don't mind.

[0xbe7a]

I took a look at how conda-forge-ci-setup does it, and they parse the variant file and use the channel_targets field to determine the upload channel. Since the variant file is contained in the info/recipe folder of the package, we can determine the target channel directly from the package

[wolfv]

great, will merge!

Just a side note: I think at some point we should refactor all the upload methods to return proper errors because some people might want to use the methods in a library (or from Python, when we have the bindings). But for now, miette does a nice job :)