Is it possible to interpolate shell output into a string?
Gentoo has changed their distfiles mirrors to use a BLAKE2B filename hash as a directory component (treewide: fix invalid gentoo/distfiles urls by ztzg · Pull Request #257808 · NixOS/nixpkgs · GitHub). We already have a BLAKE2B implementation in nixpkgs. So if we could use that to construct URLs for fetchurl, the Gentoo mirrors would be much more useful.