I know that I can tell conda to build from github as:
source:
  fn:  v1.6.0.tar.gz
  url: https://github.com/SciTools/iris/archive/v1.6.0.tar.gz
  md5: 24c2d55758325a8f05aadf99748c0d2a
or
source:
  git_url: git@github.com:SciTools/iris.git
  git_tag: v1.6.0
but how would one tell conda to build from a github branch?