Update conda recipe to use conda-only packages

[jaimergp]

Our current conda recipe relies on a dirty hack on build.sh and bld.bat to install pip-only dependencies (and even some of them are already available on conda or conda-forge). This works with conda-build<=3.16, but from 3.17 and above will fail because pip installations are deliberately blocked. For now, I have pinned conda-build to 3.16 in the CI configurations, but it'd be better to switch to a conda-only installation for the recipe (pip installations would be unaffected).