.github: add an 'apt update' step to CI

.github/workflows/ci.yml

@@ -9,6 +9,7 @@ jobs:
       - uses: actions/checkout@v3
       - name: Install Build Dependencies
         run: |
+          apt update
           sudo apt-get install cmake gettext libgtk-3-dev libappindicator3-dev libxi-dev
       - name: Build
         run: |