make Infra.toml path configurable for twoliter commands that use it

[webern]

Bottlerocket has been fine because it is using twoliter make and can change the Infra.toml path via the env var in the twoliter's Makefile.toml.

However for OOTB builds using the proper Twoliter interface, users will need to be able to specify the location of Infra.toml. Affected commands

• twoliter build variant
• twoliter build kit
• twoliter publish kit

Are twoliter fetch and twoliter update affected? (I don't think so.)