dont set IMGPKG_ANON when no auth is explicitly given

[cppforlife]

removal of this allow imgpkg to use other auth methods; this is inline with allowing git fetcher to use its regular auth mechanisms even though they are not explicitly called out in vendir.yml

[davidreuss]

Works like a charm! ⭐

❯ ~/Code/vmware-tanzu/carvel-vendir/vendir sync
Fetching: vendor + dreuss/imgpkg-demo (imgpkgBundle from gcr.io/$gcr_private_project/imgpkg-demo:test)

Lock config

apiVersion: vendir.k14s.io/v1alpha1
directories:
- contents:
  - imgpkgBundle:
      image: gcr.io/$gcr_private_project/imgpkg-demo@sha256:a05cd4917f9b48b57f872feeaeee2adffe746713bffad5ce4fa0e5c55e964b35
      tag: test
    path: imgpkg-demo
  path: vendor
kind: LockConfig

Succeeded

Slack thread on carvel channel in kubernetes slack workspace for reference.