Removes NIFs built for another architecture before a firmware build
bb_nsk.add_display puts this in front of mix firmware:
firmware: ["bb_nsk.prune_nifs", "firmware"]See BB.NSK.MixHelpers for why it is needed. Short version: every build
shares one deps/emerge/priv/native, so a host mix test leaves an x86_64
library where the firmware build can see it, and Nerves' release scrub rejects
the whole image over it.
Does nothing on the host.