Using a pkgs.writeText causes an import at evaluation time instead of
just build time. This means that no host running `nix flake check` can
check all configurations if you have mixed architectures in a flake.
For some reason I've been getting away with this. This stopped when
switching to nixos-2311. Move the known hosts with a single key into the
NixOS config directly and put the GitHub keys in a real file. These
can't go into `.knownHosts` directly as it only supports one key per
host (sigh).
Reference: https://github.com/NixOS/nix/issues/4265
