ci: update APT cache key

.github/workflows/testing.yml

@@ -39,7 +39,7 @@ jobs:
         path: /var/cache/apt/archives
         key: ${{ runner.os }}-apt-${{ hashFiles('**/apt-packages.txt') }}
         restore-keys: |
-          ${{ runner.os }}-apt-
+          ${{ runner.os }}-apt-packages
 
     - name: Install sharkd (marker replay engine)
       run: |