Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/run-tests.yaml

@@ -8,7 +8,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checking out faucetagent source
-        uses: actions/checkout@v2
+        uses: actions/checkout@v6
       - name: Installing dependencies
         run: |
           # Unfortunately this is really slow: