Because it makes full integration with gopherjs impossible (without
polluting the user's GOPATH), as long as
gopherjs/gopherjs#415 is not fixed.
Also it is kind of antithetical with the point of make.go anyway.
We still rely on CAMLI_MAKE_USEGOPATH for the integration tests that run
make.go to know that they shouldn't recursively create another temp
GOPATH (when they're already in such a temp dir, because they're started
through devcam test).