cmd/go: list_constraints test broken

[jayconrod]

The list_constraints test was introduced in CL 209660. It sets GOOS to darwin without setting GOARCH. The test is broken on builders for architectures that where darwin is not a valid GOOS.

Thanks to @cherrymui for calling this out.

[gopherbot]

Change https://golang.org/cl/209900 mentions this issue: cmd/go: fix list_constraints test on non-amd64 platforms