PROJ grids not found on Windows

[rhugonnet]

Windows has an issue with its PROJ environment variable (PROJ_DATA or PROJ_LIB) in the CI. To fix this, #304 unsets the variables which is then reset by rasterio (see https://rasterio.readthedocs.io/en/latest/faq.html#why-can-t-rasterio-find-proj-db-rasterio-from-pypi-versions-1-2-0).

This works for all OS for importing rasterio and using pyproj. However, it fails when trying to locate the grids in test_dem.py. More needs to be investigated here...