Use a fixed output derivation when unpacking input archive.

[benpye]

Whilst the output for xilinx-ise-unwrapped should be fixed, as it is not a fixed output derivation it is currently rebuilt whenever 7zz or unzip are rebuilt as these are used to unpack. We should be able to used a fixed output derivation to avoid this rebuilds, unfortunately this results in the store path for the unwrapped distribution being dumped, which is very large. If NixOS/nix#5854 is resolved for Nix then we will be able to move to a fixed output derivation.