Directory may already exist within the cache.

script/ci_install.sh

@@ -33,7 +33,7 @@ then
         git clone --depth=1 -b ${LLVM_RELEASE} https://github.com/llvm-mirror/libcxx.git llvm-source/projects/libcxx
         git clone --depth=1 -b ${LLVM_RELEASE} https://github.com/llvm-mirror/libcxxabi.git llvm-source/projects/libcxxabi
 
-        mkdir build && cd build
+        mkdir -p build && cd build
 
         cmake -DCMAKE_C_COMPILER=${CC} \
                 -DCMAKE_CXX_COMPILER=${CXX} \