CI: Add autoremove before building on Debian

This prevents spurious broken builds when the container base image
contains packages that are not needed anymore after the update, as
happened recently with the `usr-is-merged` package on Trixie.

.github/workflows/ci.yml

@@ -92,6 +92,7 @@ jobs:
       run: |
         apt-get update
         apt-get dist-upgrade -y
+        apt-get autoremove -y --purge
         apt-get install -y --no-install-recommends ca-certificates git sudo
 
     - name: Configure git in container