Enable building only some outputs of conda recipes

[vyasr]

This request comes from rapidsai/cudf#11119. The desire is to be able to build specific outputs of a multi-output recipe while skipping others.

I don't know if this is fundamentally something RAPIDS should be handling or if it is more of a request for conda itself, but it's something to consider. We should also see what impacts #47 would have on this. I don't know what the current state of support for multi-output recipes is in rattler-build, but it appears that there is at least some support for it already (see this example). Whether it covers all the use cases we need, and whether it would help enabling this particular request, remains to be seen.

[jakirkham]

Yeah would recommend raising a conda-build issue or in the case of #47 a rattler-build issue