Fix CI (#154)

.github/workflows/rust.yml

@@ -17,6 +17,9 @@ jobs:
     - uses: actions/checkout@v2
     - uses: Swatinem/rust-cache@v1
 
+    - name: Update package manager
+      run: sudo apt-get update
+
     - name: Install dependencies
       run: sudo apt-get install -y libsfml-dev libcsfml-dev