when building with CMake, there is a use case where the $BUILDIR is not created yet, when `reloc/build_rpm.sh` is launched. in order to enable us to run this script without creating $BUILDIR first, let's create this directory first. Signed-off-by: Kefu Chai <kefu.chai@scylladb.com> Closes scylladb/scylladb#18464