In go 1.11, go commands will use `GOFLAGS` as default flags, see
https://golang.org/doc/go1.11#go_command.

There is no need to pass GOFLAGS to $goflags, and if we do, go commands
will fail with "duplicate flags" error, e.g.

```
$ make test-integration WHAT=./test/integration/scheduler GOFLAGS="-v"
...
go test: v flag may be set only once
run "go help test" or "go help testflag" for more information
...
```