build-system/gnu: Support parallel builds and tests.

* guix/build/gnu-build-system.scm (build): Add `parallel-build?'
  parameter; honor it and $NIX_BUILD_CORES.
  (check): Add `parallel-tests?' parameter; likewise.

* guix/build-system/gnu.scm (gnu-build): Add `parallel-build?' and
  `parallel-tests?' parameters.
  [builder]: Inherit them.