Fix libvips install on GitHub actions

Prevent error of the form:

  Failed to fetch mirror+file:/etc/apt/apt-mirrors.txt/pool/main/p/poppler/libpoppler-glib-dev_22.02.0-2ubuntu0.2_amd64.deb  404  Not Found

.github/workflows/tests.yml

@@ -143,6 +143,7 @@ jobs:
 
     - name: Install libvips package for image processing
       run: |
+        sudo apt-get update
         sudo apt-get install libvips-dev
 
     - name: Install audiowaveform package