Bump actions/checkout from 4 to 5 (#26)

Author: dependabot[bot]

.github/workflows/tests.yml

@@ -17,7 +17,7 @@ jobs:
     container: ghcr.io/fenics/dolfinx/dolfinx:nightly
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@v5
 
       - name: Install FEniCSx-Shells
         run: |