CI: Don't install Souffle unless needed

Author: ecstatic-morse

.github/workflows/ci.yml

@@ -63,6 +63,7 @@ jobs:
           fetch-depth: 1
 
       - name: Install Souffle
+        if: contains(matrix.features, 'polonius-souffle')
         run: |
           curl -s https://packagecloud.io/install/repositories/souffle-lang/souffle/script.deb.sh | sudo bash
           sudo apt-get install souffle