ci/linux: don't install python3-testresources

If I remember correctly, this was required by the tool that makes our
AppImage, which doesn't seem to require it anymore.

.github/workflows/ci.yml

@@ -198,7 +198,7 @@ jobs:
       - name: Install system dependencies
         run: |
           sudo apt update
-          sudo apt install libgirepository1.0-dev gir1.2-ayatanaappindicator3-0.1 libayatana-appindicator3-1 python3-testresources
+          sudo apt install libgirepository1.0-dev gir1.2-ayatanaappindicator3-0.1 libayatana-appindicator3-1
 
       - name: Install appimage-builder
         run: |