ggcr: authn/kubernetes doesn't handle schemes properly

[dprotaso]

Describe the bug

Knative 1.2 pulled in the newer k8schain that uses authn/kubernetes (v0.8.1-0.20220120151853-ac864e57b117).

We got a report that it doesn't work with GitLab knative/serving#12642

It looks like .docker/config.json does allow for schemes (ie. http, https) as part of the keys - but this package doesn't handle this or normalize against the host names.

{
  "https://gcr.io": {
    "auth": "...",
    "email": "not@val.id"
}