nix: build using gcc8

Workaround for https://github.com/NixOS/nix/issues/3300.

https://hydra.nixos.org/build/109304568

    running tests
    checking Nixpkgs on i686-linux
    error: stack overflow (possible infinite recursion)
    build time elapsed:  0m0.068s 0m0.029s 0m36.549s 0m6.187s
    builder for '/nix/store/dr5kd28msqmqk3hkz0ayx10dww6s8dn9-nixpkgs-tarball-20.03pre207974.16c66591.drv' failed with exit code 1