x42-plugins: Hack to attempt fixing the perpetual build problem

Since basically forever, it randomly fails with
   do not know how to unpack source archive /nix/store/d821jkm8bgkdcv924nk7qr1q06l9is35-x42-plugins-20170428.tar.xz
on Hydra.

https://hydra.nixos.org/build/62793688