ci: always use tag for LLVM checkout

Rather than trying to match the apt installed clang version, which is
prone to intermittent issues. i.e #33345.

Github-Pull: #33364
Rebased-From: b736052e39

ci/test/01_base_install.sh

@@ -55,10 +55,9 @@ if [ -n "$PIP_PACKAGES" ]; then
 fi
 
 if [[ -n "${USE_INSTRUMENTED_LIBCPP}" ]]; then
+  ${CI_RETRY_EXE} git clone --depth=1 https://github.com/llvm/llvm-project -b "llvmorg-21.1.1" /llvm-project
+
   if [ -n "${APT_LLVM_V}" ]; then
-    ${CI_RETRY_EXE} git clone --depth=1 https://github.com/llvm/llvm-project -b "llvmorg-$( clang --version | sed --silent 's@.*clang version \([0-9.]*\).*@\1@p' )" /llvm-project
-  else
-    ${CI_RETRY_EXE} git clone --depth=1 https://github.com/llvm/llvm-project -b "llvmorg-21.1.0" /llvm-project
 
     cmake -G Ninja -B /clang_build/ \
       -DLLVM_ENABLE_PROJECTS="clang" \