specs: don't rely on possibly missing doc directory.
specs: don't rely on possibly missing doc directory.

* specs/mkrockspecs_spec.yaml: Use m4 as a temporary directory
with no other `.lua` files in it.

Signed-off-by: Gary V. Vaughan <f9023000f29773649f3850298becb9544b5fd6a9@gnu.org>

gvvaughan/slingshot