.github: checkout source code to perform cache clean up

When performing a golang cache cleanup we need to fetch the source code
so that we can retrieve the right hashfile from '**/go.sum' otherwise it
will always be empty.

Signed-off-by: André Martins <andre@cilium.io>

.github/workflows/ci-images-cache-cleaner.yaml

@@ -36,6 +36,12 @@ jobs:
           - name: docker-plugin
 
     steps:
+      # Fetch the source code so that we can get the right cache key
+      - name: Checkout Source Code
+        uses: actions/checkout@8e5e7e5ab8b370d6c329ec480221332ada57f0ab # v3.5.2
+        with:
+          persist-credentials: false
+
       # Load Golang cache build from GitHub
       - name: Load ${{ matrix.name }} Golang cache build from GitHub
         uses: actions/cache@88522ab9f39a2ea568f7027eddc7d8d8bc9d59c8 # v3.3.1