screenfetch does not work on Darwin

[zarelit]

Issue description

screenfetch is a bash script designed to run as-is on most FHS compliant linux distro and on MacOS.

The derivation does not evaluate on Mac because of procps dependency, but even when that dependency is removed the script is still wrapped and cannot access system binaries to gather MacOS info.

Tested both on Sierra and High Sierra

Steps to reproduce

nix-env -iA nixpkgs.screenfetch

[zarelit]

@matthewbauer Thank you for being so quick! This is also helpful to me to learn how to package cross platform, thank you again