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

[webern]

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).

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          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