https://hydra.nixos.org/build/126812502 is failing
you can know the revision of nixpkgs by checking https://hydra.nixos.org/build/126812502#tabs-buildinputs

for the configuration, top menu, jobset -> overview, then the Configuration tab https://hydra.nixos.org/jobset/nixos/release-20.09#tabs-configuration

there's "Nix expression:" in the Configuration tab
-> that's the thing you can nix-build

nix-build ./nixos/release-combined.nix -A nixos.tests.ejabberd.x86_64-linux