ci: Use ninja in build-clang.sh

Now that we have brought ninja back to the installed base packages, we can
use it for the initial Clang build as well.

Author: ishitatsuyuki

src/ci/docker/host-x86_64/dist-x86_64-linux/build-clang.sh

@@ -25,6 +25,7 @@ INC="/rustroot/include:/usr/include"
 # disable them. BOLT is used for optimizing LLVM.
 hide_output \
     cmake ../llvm \
+      -GNinja \
       -DCMAKE_C_COMPILER=/rustroot/bin/gcc \
       -DCMAKE_CXX_COMPILER=/rustroot/bin/g++ \
       -DCMAKE_BUILD_TYPE=Release \
@@ -39,8 +40,8 @@ hide_output \
       -DLLVM_ENABLE_PROJECTS="clang;lld;compiler-rt;bolt" \
       -DC_INCLUDE_DIRS="$INC"
 
-hide_output make -j$(nproc)
-hide_output make install
+hide_output ninja
+hide_output ninja install
 
 cd ../..
 rm -rf llvm-project