code: public ProcessRunner withEnvironmentVariable(String name, String value) {
      _env.put(name, value);
      return this;
    }
docs: Adds a name-value pair into this process' environment.
  This can be called multiple times in a chain to set multiple
  environment variables.
  
  @param name the variable name
  @param value the variable value
  @return this object for chaining
  
  @see     ProcessBuilder
  @see     System#getenv()