package/ci: use all cores when building with MinGW.
package/ci: use all cores when building with MinGW.

Oh, so that's why it was so slow.

MiUishadow/magnum,MiUishadow/magnum,MiUishadow/magnum,MiUishadow/magnum,MiUishadow/magnum,MiUishadow/magnum