Fixes #5643: add support for environment variable through ${node.env[XXX...

...]} notation
I don't understand how this is intended to be used. Should users write out variables names like ```${get_environement_variable.env[LANG]```? Or does the "node.env" allow this to define a global var in a different context? If so, why are we not using ```^context``` as described in https://docs.cfengine.com/docs/3.6/reference-promise-types-commands.html#module?