only install llvm if it is not already installed

Signed-off-by: reubenmiller <reuben.d.miller@gmail.com>

Author: reubenmiller

mk/install-build-tools.sh

@@ -227,7 +227,7 @@ linux*)
   fi
   ;;
 darwin*)
-  brew install llvm
+  brew list llvm >/dev/null 2>&1 || brew install llvm
   ;;
 esac