CI: we should support generating a build matrix for *only* the oldest and newest supported kernels

What our CI should do is generate a build matrix to validate PRs that includes _only_

- Oldest and newest supported kernel
- `host` and `zone` variant
- `x86_64` and one other arch variant (at least).

So - 6 kernel builds in total.

but the existing matrix gen script doesn't allow the above to be expressed exactly (AFAICT).