Change ^ behaviour to be union

[thesamesam]

^ should probably be a union of the above package's keywords and the current keywords, to avoid accidentally stabling a new version of a dependency without including all of the arches for which it is currently stable.

For example, in bug 835772, prusaslicer needed a newer TBB, but prusaslicer is only stable on a subset of TBB's arches. When using ^ to fill the package list, TBB is then only going to be stabilised for the intersection of prusaslicer and TBB's arches, even though we really want all of TBB's existing stable arches to be included too.