The name of an environment variable that the parameter value will be read from, if it was omitted from the command-line. An error will be reported if the environment value cannot be parsed.
readonly environmentVariable: string | undefined;
The environment variable name must consist only of upper-case letters, numbers, and underscores. It may not start with a number.
This feature cannot be used when IBaseCommandLineDefinition.required is true, because in that case the environmentVariable would never be used.