x/build: set GOTRACEBACK=2 on all builders?

[minux]

Some of the bugs are fairly hard to reproduce, and without
the system function stacks, it's difficult to see the problem.

For example, #15145.

I propose that we set GOTRACEBACK=2 on all builders.

[bradfitz]

Yes, let's just do this.

[randall77]

It's now spelled GOTRACEBACK=system. (2 still works, but is deprecated.)