Add env var to check if inside a cross container

[Emilgardis]

Checklist

• I've looked through the issues and pull requests for similar request
• This feature could be solved with a custom docker image (optional)

Describe your request

cross can set a environment variable CROSS_INSIDE to tell users that the context the build it's running is within a cross container.

Describe why this would be a good inclusion for cross

Useful for special cases when using bindgen, etc etc

[Threated]

There seems to be a CROSS_RUNNER env var which is used for some cross internals, which I found will facing the same issue.
I just check if its set or not.