This worked fine in the PR introducing the parallel build of these
tools, but now fails, at least sometimes.

The reason is simple, ~2.4 GB of available memory for Shippable, and
compilation of d_do_test alone requiring about 1.7 GB on my Win64
machine (a bit more for the unittests). -lowmem works very well here,
reducing it to 0.5-0.6 GB.