ci: Use checkout action v6

Just for consistency.

Author: jonasfreimuth

.github/workflows/ci.yml

@@ -11,7 +11,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Check out the code
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
 
       - name: Setup pixi
         uses: prefix-dev/setup-pixi@v0