I think that under US law you can legally clone it from github [1] and build/run it [2] without a license.

[1] GitHub has a valid license to create copies by virtue of their TOS. You aren't creating a copy, github is, and they are then lawfully transferring it to you.

[2] This is explicitly called out as something you can legally do without a license in copyright law. "All rights reserved" is just the lack of a license and doesn't prevent you from doing things that weren't illegal in the first place.: https://www.law.cornell.edu/uscode/text/17/117