envtest: Stick to a specific setup-envtest version

Using `@latest` for tools dependecies produces unrepeatable
builds which might break at any point in time. This is
particularly important when maintaining released versions
which need a backport fix.

Note: At the moment is not possible to reference the package
`sigs.k8s.io/controller-runtime/tools/setup-envtest` with a specific
version:

```
go: sigs.k8s.io/controller-runtime/tools/[email protected]: module sigs.k8s.io/[email protected] found, but does not contain package sigs.k8s.io/controller-runtime/tools/setup-envtest
```

See kubernetes-sigs/kubebuilder#2480

Ref:
openshift#849
Signed-off-by: Andrea Panattoni <[email protected]>