explicitly use built protoc for CMake build
diff --git a/CMakeLists.txt b/CMakeLists.txt
index a607864..a0ac4f6 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -304,7 +304,7 @@
              "${_gRPC_PROTO_GENS_DIR}/${RELFIL_WE}_mock.grpc.pb.h"
              "${_gRPC_PROTO_GENS_DIR}/${RELFIL_WE}.pb.cc"
              "${_gRPC_PROTO_GENS_DIR}/${RELFIL_WE}.pb.h"
-      COMMAND ${_gRPC_PROTOBUF_PROTOC}
+      COMMAND $<TARGET_FILE:${_gRPC_PROTOBUF_PROTOC}>
       ARGS --grpc_out=generate_mock_code=true:${_gRPC_PROTO_GENS_DIR}
            --cpp_out=${_gRPC_PROTO_GENS_DIR}
            --plugin=protoc-gen-grpc=$<TARGET_FILE:grpc_cpp_plugin>