CI: update configured sources for package management

Signed-off-by: Iker Pedrosa <[email protected]>

.github/workflows/build.yml

@@ -7,6 +7,7 @@ jobs:
     - uses: actions/checkout@v2
     - name: Install dependencies
       run: |
+        sudo apt-get update -y
         sudo apt-get install graphviz plantuml
         pip3 install virtualenv
         virtualenv .venv