Sourcing bash from `/bin/bash` doesn't work on NixOS, and on other platforms it ends up using the system-default bash rather than the one sourced by Nix.
/usr/bin/env is a utility which will find the related command by searching PATH, this allows for nix to introduce other interpreters. It's also considered a "best practice" as some systems such as android, solaris, and other may also not put their default interpreters in `/bin`
Depends on D3274