I think that there is a bug in conda setup. There is a branch dedicated to fix the issue. (
https://github.com/google/skywater-pdk/tree/fix-conda-channels).
For what is worth the way I managed to fix that on my local copy was to:
1. source open_pdks/sources/sky130-pdk/env/conda/bin/activate
2. conda install --name skywater-pdk-scripts -c litex-hub yosys
Then I re-ran make.