CI: update and pin actions/setup-python v6.0 (#3861)

Author: mmetc

.github/workflows/docker-tests.yml

@@ -61,7 +61,7 @@ jobs:
           working-directory: "./docker/test"
 
       - name: "Set up Python"
-        uses: actions/setup-python@v5
+        uses: actions/setup-python@e797f83bcb11b83ae66e0230d6156d7c80228e7c
         with:
           python-version-file: "./docker/test/.python-version"