Install clang from Ubuntu and push image from buildx

apt.llvm.org has no repository for the current ubuntu:rolling release,
and nothing pins clang 20 - CC=clang already resolved to the distro
clang. Drop the llvm.sh step along with its helper packages.

docker build now runs under the buildx docker-container driver, which
keeps the result in the build cache unless told otherwise, so push
directly from the build instead of tagging locally.

.gitea/workflows/ci.yml

@@ -35,9 +35,7 @@ jobs:
               docker push "$image:$latest"
             fi
           else
-            docker build -t "$image:$tag" -t "$image:$latest" .
-            docker push "$image:$tag"
-            docker push "$image:$latest"
+            docker build --push -t "$image:$tag" -t "$image:$latest" .
           fi
 
   pre-commit: