Allow Instructors to feed in environment variables to Tango Autograding Instance

[20wildmanj]

Dual Autolab / Tango feature:

15-462 uses an autograder in Gradescope with a BUILD_MODE environment variable. When this variable is. set Then, after the deadline upload the instructors modify the autograder with build mode false, reupload the autograder, and run the test suite. This could be improved by having a way to specify an environment variable and value so that a reupload of the autograder wouldn't be necessary.