a couple of CI configuration fixes
Conda packaging
We previously restricted conda package uploading to master branch on the main Kwant repository, as otherwise this step would fail for forked repositories without a valid anaconda key. Now we build the conda packages and upload to anaconda in two separate jobs, and the upload job is only run on the main Kwant repository.
SSH Host Keys
See 2nd commit message for details -- we now ignore the host key of the web server where we upload the docs