[apple] workaround the swift-ci linux failure for PR bots

The swift-ci linux PR bot is failing because it's not able
to build libcxx fuzzer becuase of missing libcxxabi. It shouldn't
even build the fuzzer, it only tries to because of the monorepo, which
has libcxxabi next to compiler-rt.

Author: hyp

compiler-rt/CMakeLists.txt

@@ -476,7 +476,7 @@ if(COMPILER_RT_USE_LIBCXX)
   endforeach()
   foreach(path IN ITEMS ${LLVM_MAIN_SRC_DIR}/projects/libcxxabi
                         ${LLVM_MAIN_SRC_DIR}/runtimes/libcxxabi
-                        ${LLVM_MAIN_SRC_DIR}/../libcxxabi
+                        # Hide monorepo libcxxabi path for swift-ci: ${LLVM_MAIN_SRC_DIR}/../libcxxabi
                         ${LLVM_EXTERNAL_LIBCXXABI_SOURCE_DIR})
     if(IS_DIRECTORY ${path})
       set(COMPILER_RT_LIBCXXABI_PATH ${path})