Make `--channels` also affect mulled channels and update/extend howto use bioconda artifacts

[bernt-matthias]

Adapted --channels such that it also affects Galaxy's mulled channels config.

Update docs due to the recent change in bioconda CI (packages and images are now available only as download)

Also document how to set conda channels for mulled container building (see also https://github.com/galaxyproject/galaxy/pull/13755)