Tie `ENVTEST_K8S_VERSION` to the version of `k8s.io/client-go`

[joelanford]

Something like this: https://github.com/operator-framework/helm-operator-plugins/blob/dd32bac92534db6a00fe30252cd944c361639ec5/Makefile#L41-L46

Originally posted by @joelanford in #81 (comment)

[joelanford]

Closed by #103