docs: @Override
      public String lookup(String key) {
          final String value = System.getenv(key);
  
          if (value == null && strict) {
              throw new UndefinedEnvironmentVariableException("The environment variable '" + key
                      + "' is not defined; could not substitute the expression '${"
                      + key + "}'.");
          }
  
          return value;
      }