ci: Remove pixi version to avoid issues with the new pixi.lock format

Author: jorisv

.github/workflows/macos-linux-windows-pixi.yml

@@ -92,7 +92,6 @@ jobs:
 
       - uses: prefix-dev/setup-pixi@v0.8.1
         with:
-          pixi-version: v0.33.0
           cache: true
           environments: ${{ matrix.environment }}