ci: add lld-15 to PR builder docker image

Author: lqd

src/ci/docker/host-x86_64/x86_64-gnu-llvm-15/Dockerfile

@@ -17,6 +17,7 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
   gdb \
   llvm-15-tools \
   llvm-15-dev \
+  lld-15 \
   libedit-dev \
   libssl-dev \
   pkg-config \