How to set variables across several Earthfiles with earthly for continuous integration

Question

If you have a repository like that:

my_project
|
|-- Earthfile
|-- src
|     |-- main.cpp
|-- include
|     |-- header.hpp
|-- docs
|     |-- Earthfile
|     |-- index.rst
|-- test
      |-- Earthfile
      |-- gtest_feature00.cpp

how would-you share variable between Earthfile (compiler version for format, build and tests for instance) ?