CI: run apt-get update before installing libvips (#538)

Author: kleisauke

.github/workflows/ci.yml

@@ -23,7 +23,9 @@ jobs:
           python-version: ${{ matrix.python-version }}
 
       - name: Install libvips
-        run: sudo apt-get install --no-install-recommends libvips
+        run: |
+          sudo apt-get update
+          sudo apt-get install --no-install-recommends libvips
 
       - name: Lint with flake8
         run: |