Add comments about the missing $CFGUARD_ARGS when building sanitizers

Dockerfile.dev

@@ -132,6 +132,7 @@ RUN cd test && \
 
 # Build sanitizers. Ubsan includes <typeinfo> from the C++ headers, so
 # we need to build this after libcxx.
+# $CFGUARD_ARGS is intentionally omitted here.
 RUN ./build-compiler-rt.sh $TOOLCHAIN_PREFIX --build-sanitizers
 
 # Sanitizers on windows only support x86.