add a default system in case system = [] for *only* universal flakes

[maxheld83]

In #19 I mistakenly created a flake which makes flake-iter return system = [] which breaks the CI.

(In DeterminateSystems/flake-iter#21, I let flake-iter emit a warning for these circumstances).

However, I wondered whether it was theoretically possible to legitimately end up with system = [], as, for example, when your flake only has a schema and a pdf output.

I'm not experienced with nix/flakes enough to know if/how plausible this scenario is.

If it is plausible, I'd be happy to write a PR which lets the CI job default to some system with, say '{"x86_64-linux": "ubuntu-latest"}', just so the matrix job as something to run on.