x/build/cmd/gopherbot: automatically assign OS-* labels to new issues

[mvdan]

For example, if a new issue title only mentions one operating system name (Linux, Windows, Mac, OSX, Solaris, etc), it should add the label.

I think this should only apply to new issues, as sometimes it turns out that issues affect more or less operating systems than initially thought. And the heuristic might be wrong in some edge cases, so we want a human to be able to override it.

I also think this should be a regex, or at the very least a case insensitive string search. For example, I'd imagine that all of darwin, os x, OSX, and Mac should map to OS-Darwin.