CI: use setup-pixi in test jobs

Author: lucascolley

.github/workflows/ci.yml

@@ -61,6 +61,11 @@ jobs:
           python-version: ${{ matrix.python-version }}
           allow-prereleases: true
 
+      - uses: prefix-dev/setup-pixi@v0.8.0
+        with:
+          pixi-version: v0.30.0
+          cache: true
+
       - name: Install package
         run: python -m pip install .[test]