Default to machine architecture for build variant, build kit, fetch and publish kit

Currently `twoliter build variant` and `twoliter build kit` default to `x86_64`. Instead they should default to the machine architecture (for example the architecture that `twoliter` was built for).

---
Can we default to the arch that `twoliter` was built for? It'd be sort of surprising to get x86_64 builds by default on an aarch64 host.

_Originally posted by @bcressey in https://github.com/bottlerocket-os/twoliter/pull/249#discussion_r1622536784_

Start with the `twoliter build variant`, `twoliter build kit`, fetch, and publish kit entry points and trace how their architecture defaults are selected. Verify the machine architecture available to the built `twoliter` is used instead of `x86_64`, and confirm all four commands behave accordingly.

Written by the indexing model from the issue text.