Use mold in CI.

This is faster than the normal GNU ld when compiling yk.

.buildbot_dockerfile_debian

@@ -3,12 +3,14 @@ ARG CI_UID
 RUN useradd -m -u ${CI_UID} ci
 RUN apt-get update && \
 apt-get -y install clang-15 make curl procps file git cmake python3 \
-    libtinfo-dev libzip-dev ninja-build
+    libtinfo-dev libzip-dev mold ninja-build
 WORKDIR /ci
 RUN update-alternatives --install /usr/bin/cc cc /usr/bin/clang-15 999
 RUN update-alternatives --set cc /usr/bin/clang-15
 RUN update-alternatives --install /usr/bin/c++ c++ /usr/bin/clang++-15 999
 RUN update-alternatives --set c++ /usr/bin/clang++-15
+RUN update-alternatives --install /usr/bin/ld ld /usr/bin/mold 999
+RUN update-alternatives --set ld /usr/bin/mold
 RUN ln -sf /usr/bin/clang-15 /usr/bin/clang
 RUN ln -sf /usr/bin/clang++-15 /usr/bin/clang++
 RUN chown ${CI_UID}:${CI_UID} .