Use git remote-base instead of zip-archive for cloud image-update example

[stealthybox]

Kustomize doesn't support local bases in sibling directories from the same archive.
( go-getter limitation: kubernetes-sigs/kustomize#2538 (comment) )

Quick fix for this broken example while we work on additional docs to help people fork into their repos.
@stefanprodan @kingdonb

problem:

# building /_base via zip works
kustomize build https://github.com/fluxcd/flux2/archive/main.zip//flux2-main/manifests/integrations/registry-credentials-sync/_base

# building /azure via zip fails because of the sibling dependency
kustomize build https://github.com/fluxcd/flux2/archive/main.zip//flux2-main/manifests/integrations/registry-credentials-sync/azure
Error: accumulating resources: 2 errors occurred:
        * accumulateFile error: "accumulating resources from '../_base': evalsymlink failure on '/tmp/kustomize-462235843/_base' : lstat /tmp/kustomize-462235843/_base: no such file or directory"
        * loader.New error: "error loading ../_base with git: url lacks host: ../_base, dir: evalsymlink failure on '/tmp/kustomize-462235843/_base' : lstat /tmp/kustomize-462235843/_base: no such file or directory, get: invalid source string: ../_base"

solution:

# build via git
kustomize build git@github.com/fluxcd/flux2//manifests/integrations/registry-credentials-sync/azure

[stealthybox]

Also, the previous example was broken as well -- forget the top-level //flux2-main/... folder in the archive path