Simplify nix shell environment
Simplify nix shell environment

`buildFHSUserEnv` uses `chroot` to create a kind of sandbox environment.  This
change uses a simpler mechanism, and also provides the necessary libraries for
running all the demos.

AlexIljin/factor,tgunr/factor,mrjbq7/factor,tgunr/factor,AlexIljin/factor,tgunr/factor,tgunr/factor,mrjbq7/factor,mrjbq7/factor,factor/factor,bpollack/factor,mrjbq7/factor,bpollack/factor,bpollack/factor,bpollack/factor,bpollack/factor,factor/factor,AlexIljin/factor,factor/factor,mrjbq7/factor,factor/factor,tgunr/factor,AlexIljin/factor,bpollack/factor,factor/factor,mrjbq7/factor,AlexIljin/factor,bpollack/factor,AlexIljin/factor,AlexIljin/factor,tgunr/factor,factor/factor