Merge bitcoin/bitcoin#26574: ci: use ci_exec_root for clang install

54dd8f51ce ci: use ci_exec_root for clang install (josibake)

Pull request description:

  fixes a bug introduced in #25900 ; see https://github.com/bitcoin/bitcoin/pull/25900#issuecomment-1327311069

  the general idea of #25900 was to use a non-root user as much as possible to avoid modifying the user's local filesystem. however, it appears the root user is needed to correctly install clang.

ACKs for top commit:
  hebasto:
    ACK 54dd8f51ce, tested on Ubuntu 22.04.

Tree-SHA512: beb01d4b6127fbba3c8d18e85cf7ec7d1b2ec93ea05c475ab51bcaa04ef1b0591d886f1a7e0732c5ae86806013f022c0b44027380d2b0cfb1bfdc843e40f99b4

ci/test/04_install.sh

@@ -130,8 +130,8 @@ fi
 CI_EXEC mkdir -p "${BASE_SCRATCH_DIR}/sanitizer-output/"
 
 if [[ ${USE_MEMORY_SANITIZER} == "true" ]]; then
-  CI_EXEC "update-alternatives --install /usr/bin/clang++ clang++ \$(which clang++-12) 100"
-  CI_EXEC "update-alternatives --install /usr/bin/clang clang \$(which clang-12) 100"
+  CI_EXEC_ROOT "update-alternatives --install /usr/bin/clang++ clang++ \$(which clang++-12) 100"
+  CI_EXEC_ROOT "update-alternatives --install /usr/bin/clang clang \$(which clang-12) 100"
   CI_EXEC "mkdir -p ${BASE_SCRATCH_DIR}/msan/build/"
   CI_EXEC "git clone --depth=1 https://github.com/llvm/llvm-project -b llvmorg-12.0.0 ${BASE_SCRATCH_DIR}/msan/llvm-project"
   CI_EXEC "cd ${BASE_SCRATCH_DIR}/msan/build/ && cmake -DLLVM_ENABLE_PROJECTS='libcxx;libcxxabi' -DCMAKE_BUILD_TYPE=Release -DLLVM_USE_SANITIZER=MemoryWithOrigins -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ -DLLVM_TARGETS_TO_BUILD=X86 ../llvm-project/llvm/"