build(ci): cleanup docker images in gha runner before building new im…

…ages (#2734)

Follow up to #2711 and #2727

Github keeps some image cache in the GHA runners which keeps around
base containers with vulnerabilities.

We can't use --pull because we use local-built tools image.

.github/workflows/main.yaml

@@ -104,5 +104,6 @@ jobs:
         password: ${{ secrets.GITHUB_TOKEN }}
     - run: sudo rm -rf /usr/share/dotnet
     - run: sudo rm -rf "$AGENT_TOOLSDIRECTORY"
+    - run: docker image prune -af
     - run: make release-snapshot
     - run: ./build/push_images.sh