infra/pkg-golang: enforce number of parallel jobs

By default, the go compiler will spawn as many jobs as there are CPUs
available, thus possibily over-shooting the limits set by the user.

Make it abide by the user's wish, and specify the number of jobs allowed
to run.

We can do so without fear of a package failing to build in parallel,
because they were already all building in parallel, as that is the
default for the go compiler.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>