This fixes `fetchurl-force.nix` not being installed, which breaks
bundix for some gems.
E.g.
```
$ nix-build --argstr url https://rubygems.org/gems/nio4r-2.1.0.gem /nix/store/y6959dxal86l3alc0ryf7752prbbkzxg-bundix-2.2.0/lib/ruby/gems/2.3.0/gems/bundix-2.2.0/lib/bundix/fetchurl-force.nix
error: getting status of ‘/nix/store/y6959dxal86l3alc0ryf7752prbbkzxg-bundix-2.2.0/lib/ruby/gems/2.3.0/gems/bundix-2.2.0/lib/bundix/fetchurl-force.nix’: No such file or directory
```