Default to forwarding CI environment variable

[emarsden]

I check for the CI environment variable to disable "expensive" tests (for instance that generate a lot of network activity) on the GitHub CI workers. This is always set on a GitHub runner, running natively on in a container.

It seems that it might be convenient for users to forward this environment variable into the VMs by default, or at least to make a reference to this in the README.md section on environment_variables.

[jacob-carlborg]

The CI and any environment variables starting with GITHUB_ are already automatically forwarded. I've updated the readme to reflect this, in the "Inputs" section: https://github.com/cross-platform-actions/action#inputs.