Add fftw for GalSim

.github/workflows/unittests.yml

@@ -27,6 +27,7 @@ jobs:
         python -m pip install --upgrade pip
         pip install .
         pip install pytest
+        conda install fftw
         pip install git+https://github.com/GalSim-developers/GalSim@1294_from_images
     - name: Run unit tests with pytest
       run: |