Set arduino.json output folder path with an environmental variable

[themaninthesuitcase]

Bumping #880 which was closed as stale.

It would be really helpful to be able to set the "output" variable in the arduino.json file with environmental variables (similar to: https://code.visualstudio.com/docs/editor/variables-reference#_environment-variables)

This would mean I could use the same file on multiple platforms, or have it use environment variables to build the path.