Ну как-то так:

src = builtins.fetchurl {
- url = "https://pubs.opengroup.org/onlinepubs/9699919799/download/susv4-2018.tar.bz2";
- sha256 = "0mxs2763q280rk15b41w9xvlw0s1ny46fyfzcjhrprhc69ik3mvn";
+ url = "https://www.dropbox.com/s/ovxorlv7ltmlw3k/susv4-2018.tar.bz2?dl=1";
+ sha256 = "0da71fas8dsk46vq7yl0yqa1ymnbsrfsfj09x1mr1s3b8xfpd0pf";
};