Pin (and update) actions

Updates the checkout action as this uses a deprecated Node.js version
and the old version will therefore be forced to run on node20. Further
pins actions as suggested byt OpenSSF Scorecard, see
https://github.com/ossf/scorecard/blob/main/docs/checks.md#pinned-dependencies.

.github/workflows/ci-tuner.yml

@@ -20,10 +20,10 @@ jobs:
 
     steps:
       - name: Checkout code
-        uses: actions/[email protected]
+        uses: actions/checkout@692973e3d937129bcbf40652eb9f2f61becf3332 # v4.1.7
 
       - name: Set up Python
-        uses: actions/setup-python@v4
+        uses: actions/setup-python@39cd14951b08e74b54015e9e001cdefcf80e669f # v5.1.1
         with:
           python-version: '3.10.12'