Try setting environment variable to resolve FFTW build issue

Author: matt-graham

.github/workflows/tests.yml

@@ -33,6 +33,8 @@ jobs:
           - os: macos-latest
             python-version: "3.8"
       fail-fast: false
+    env:
+      CMAKE_POLICY_VERSION_MINIMUM: 3.5
 
     steps:
     - name: Checkout Source