modules: disable modules via os.Setenv

[thepudds]

This passes travis when travis is updated to run tip, I think.

Note that changes were required to both go-fuzz-build and go-fuzz.

I started this prior to @josharian sending in #240. This PR can probably be abandoned in favor of #240, but sending in at least for contrast.

[josharian]

Thanks for sending this. I’m torn. This is short and succinct and easy to rip out later; on the other hand, this is a pretty big hammer. If the more targeted approach in #240 works, I guess I have a weak preference for it. I’m open to being convinced otherwise though. :)

[dvyukov]

Uh, sorry, I saw @josharian PR before I saw that this exists. So what are we doing now?

[thepudds]

Hi @dvyukov, makes sense to close this one. Two solutions are better than none, but all we really need is one solution. ;-)