Lookaside cache should be specified in Twoliter.toml

Currently, if a user wants to override the location of the lookaside cache, they have to do so with a BUILDSYS_ environment variable that is passed to the Twoliter build variant and build kit commands. Instead, this should be provided in the user's Twoliter.toml file.

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This feels like a `Twoliter.toml` sort of concern - letting developers specify a list of allowed caches, or at least set the cache URL globally for their project.

_Originally posted by @bcressey in https://github.com/bottlerocket-os/twoliter/pull/249#discussion_r1622540756_

Start by tracing how the BUILDSYS_ environment variable reaches the Twoliter build variant and build kit commands, then inspect the project's Twoliter.toml handling. Clarify whether the configuration should support a global cache URL or a list of allowed caches. Done means users can specify the lookaside cache in Twoliter.toml without relying on the environment variable.

Written by the indexing model from the issue text.