Fix envtest setup and usage

[aryan9600]

We currently have a a target setup-envtest which installs the etcd, kubectl , etc for users but we don’t use it implicitly in the test target. We also don't seem to set the KUBEBUILDER_ASSETS env to the dir where the binaries are installed so that they can be found during tests.

Projects:

• flux2
• pkg
• source-controller
• image-automation-controller
• image-reflector-controller
• notification-controller
• source-watcher
• kustomize-controller

[stefanprodan]

Please use https://github.com/fluxcd/helm-controller/blob/main/Makefile as an example for all others. Note that testbin must be added to gitignore. Also go get has been deprecated for binaries, we should use go install starting with Go 1.17.

[stefanprodan]

We need this for fluxcd/pkg project as well.

[stefanprodan]

Thanks @aryan9600 for taking care of this 🥇