ci: add caching for APT packages

.github/workflows/testing.yml

@@ -28,6 +28,14 @@ jobs:
         python-version: ${{ matrix.python-version }}
         cache: pip
 
+    - name: Cache APT packages
+      uses: actions/cache@v4
+      with:
+        path: /var/cache/apt/archives
+        key: ${{ runner.os }}-apt-${{ hashFiles('**/apt-packages.txt') }}
+        restore-keys: |
+          ${{ runner.os }}-apt-
+
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip