Create builtin ARG EARTHLY_VERSION

[cehoffman]

This would be useful for referencing the docker setup script to make an image suitable for use WITH DOCKER, e.g. DO github.com/earthly/lib:$EARTHLY_VERSION+INSTALL_DIND

[alexcb]

https://github.com/earthly/earthly/blob/main/variables/builtin.go#L18 is where we can add a new entry for EARTHLY_VERSION.

Perhaps we should also include EARTHLY_BUILD_SHA, which would point to the corresponding git sha that was used for the build?

Here are the relevant strings that are set at build time and would need to be propigated into builtin.go: https://github.com/earthly/earthly/blob/main/cmd/earthly/main.go#L168-L172