Fixes a small issue in the `build-go.sh` where the script failed to extract the `k8s.io/client-go` version from the `go.sum` due to 2 factors:

• Wrong path: The file `go.sum` is located inside the `go-controller` directory from the main project, the current variable prefix used to point the path `$OVN_KUBE_GO_PACKAGE` won't find the file because the script runs already in `$OVN_KUBE_ROOT`. So in essence it's trying to run this command:
  ```
  grep 'k8s.io/client-go' github.com/ovn-org/ovn-kubernetes/go-controller/go.sum
  ```
  Inside `$OVN_KUBE_ROOT`, which is the full path to the `go-controller` directory. In my case:
  ```bash
  $> echo $OVN_KUBE_ROOT
  /home/jgil/go/src/github.com/ovn-org/ovn-kubernetes/go-controller
  ```

• Possibly the wrong file to check to extract this value: Checking the contents of `go.sum` shows that the list of entries that match the `k8s.io/client-go` criteria are multiple:
  ```bash
  $> grep 'k8s.io/client-go' go.sum
  k8s.io/client-go v0.18.8/go.mod h1:HqFqMllQ5NnQJNwjro9k5zMyfhZlOwpuTLVrxjkYSxU=
  k8s.io/client-go v0.20.1/go.mod h1:/zcHdt1TeWSd5HoUe6elJmHSQ6uLLgp4bIJHVEuy+/Y=
  k8s.io/client-go v0.20.4/go.mod h1:LiMv25ND1gLUdBeYxBIwKpkSC5IsozMMmOOeSJboP+k=
  k8s.io/client-go v0.20.6/go.mod h1:nNQMnOvEUEsOzRRFIIkdmYOjAZrC8bgq0ExboWSU1I0=
  k8s.io/client-go v0.27.1 h1:oXsfhW/qncM1wDmWBIuDzRHNS2tLhK3BZv512Nc59W8=
  k8s.io/client-go v0.27.1/go.mod h1:f8LHMUkVb3b9N8bWturc+EDtVVVwZ7ueTVquFAJb2vA=
  ```
  Whereas `go.mod` contains only one entry:
  ```zsh
  $>grep k8s.io/client-go go.mod
  k8s.io/client-go v0.27.1
  ```

The difference is that without this fix, the `make build` shows this error in the first lines of execution:
```
$> make build
hack/build-go.sh cmd/ovnkube cmd/ovn-k8s-cni-overlay cmd/ovn-kube-util hybrid-overlay/cmd/hybrid-overlay-node cmd/ovndbchecker cmd/ovnkube-trace
grep: github.com/ovn-org/ovn-kubernetes/go-controller/go.sum: No such file or directory
...
...
+ env CGO_ENABLED=0 go build -v -mod vendor -gcflags '' -ldflags '
...
...
-X k8s.io/client-go/pkg/version.gitVersion=
```
And with this change, the tag
```
$> make build
hack/build-go.sh cmd/ovnkube cmd/ovn-k8s-cni-overlay cmd/ovn-kube-util hybrid-overlay/cmd/hybrid-overlay-node cmd/ovndbchecker cmd/ovnkube-trace
...
...
+ env CGO_ENABLED=0 go build -v -mod vendor -gcflags '' -ldflags
...
...
-X k8s.io/client-go/pkg/version.gitVersion=v0.27.1 '
```