Instead of `services.nixseparatedebuginfod2.substituter = "foo"`, set `services.nixseparatedebuginfod2.substituters = [ "foo" ]` (possibly with mkForce to override the default value).
'')
];
options={
services.nixseparatedebuginfod2={
enable=lib.mkEnableOption"nixseparatedebuginfod2, a debuginfod server providing source and debuginfo for nix packages";