ci: update apt sources before installing deps

Author: lucc

.github/workflows/ci.yml

@@ -18,6 +18,7 @@ jobs:
         python-version: ${{ matrix.python-version }}
     - name: Install dependencies
       run: |
+        sudo apt-get update
         sudo apt-get install -y locales-all
         python -m pip install --upgrade pip
         python -m pip install flake8