[libc++] Use $CXX instead of 'c++' in run-buildbot

We don't have `c++` anymore in the Docker image, but the script does
require $CXX to be in the environment so that should always work.

Differential Revision: https://reviews.llvm.org/D148830

Author: ldionne

libcxx/utils/ci/run-buildbot

@@ -337,7 +337,7 @@ bootstrapping-build)
           -DCMAKE_INSTALL_PREFIX="${INSTALL_DIR}" \
           -DLLVM_ENABLE_PROJECTS="clang" \
           -DLLVM_ENABLE_RUNTIMES="libcxx;libcxxabi;libunwind" \
-          -DLLVM_RUNTIME_TARGETS="$(c++ --print-target-triple)" \
+          -DLLVM_RUNTIME_TARGETS="$(${CXX} --print-target-triple)" \
           -DLLVM_TARGETS_TO_BUILD="host" \
           -DRUNTIMES_BUILD_ALLOW_DARWIN=ON \
           -DLLVM_ENABLE_ASSERTIONS=ON