The Custom-Symbolic-and-Byproduct case fails strangely on some
filesystems used by our nightly testing.  Somehow on the first build the
`use-byproduct` output ends up with a timestamp older than
`gen-byproduct-stamp` even though the build log clearly shows them build
in the correct order (and must according to build system dependencies).
Work around this problem by adding an extra delay before building
`use-byproduct`.

Tested-by: Gerhard Grimm <gerhard.grimm@detec.com>