Conda appveyor master
xref https://github.com/conda/conda/issues/6251

Closes https://github.com/pandas-dev/pandas/issues/18073


That apparently wasn't it. (We'll still want to update to the newer miniconda though). > That apparently wasn't it. (We'll still want to update to the newer miniconda though).

we already use the latest windows installeer, e.g. see ``ci/install.ps1`` superseded by #18105 