Use setup-envtest for installing the binaries required for using
envtest.
The latest envtest binaries for k8s v1.21.2 are not compatible with
kubebuilder < v3, which results in envtest suite test tear down
failure. envtest fails to stop the kube-apiserver.

```
timeout waiting for process kube-apiserver to stop
```

Pin to k8s v1.19.2 envtest binaries that work with kubebuilder v2.

Signed-off-by: Sunny <darkowlzz@protonmail.com>

Use fluxcd/pkg/actions/envtest to setup envtest.

Since the run-test action runs in a container, the host $PATH is not
passed to the test container. The ENV directive in actions/run-tests
Dockerfile is used to append the PATH with envtest binary path. This
helps avoid redownload of the envtest binaries when it's not found in
the path via the Makefile targets.

Signed-off-by: Sunny <darkowlzz@protonmail.com>