fix(ci): Inconsistent use of IMAGE_REGISTRY and IMAGE_NAMESPACE Makefile variables (#25846)

[dudinea]

Fixes #25846

This PR implements the same logic for creating full image tag in both the
Makefile (for building docker images) and hack/update-manifests.sh script:

IMAGE_REGISTRY is used for FQDN of the container registry. If it is not set, the full tag won't have the registry part. It means that the default registry for the container engine will be used (it's docker.io unless configured otherwise). If IMAGE_REGISTRY is configured while IMAGE_NAMESPACE is not (which does not have much sense) an error is generated.

IMAGE_NAMESPACE contains name of user account in the container registry.
For backward compatibility it can provide both container registry and user account (like quay.io/johnsmith). If it not set, the full tag won't have the account part.

IMAGE_REPOSITORY contains name of the image (argocd by default). Now this parameter is supported for building docker images as well.

The default image parameters for the standard image tag are specified in the Makefile
and the hacks/update-manifests.sh. ArgoCD forks may override those values using the
GithHub Actions variables.

Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this does not need to be in the release notes.
• The title of the PR states what changed and the related issues number (used for the release note).
• The title of the PR conforms to the Title of the PR
• I've included "Closes [ISSUE #]" or "Fixes [ISSUE #]" in the description to automatically close the associated issue.
• I've updated both the CLI and UI to expose my feature, or I plan to submit a second PR with them.
• Does this PR require documentation updates?
• I've updated documentation as required by this PR.
• I have signed off all my commits as required by DCO
• I have written unit and/or e2e tests for my change. PRs without these are unlikely to be merged.
• My build is green (troubleshooting builds).
• My new feature complies with the feature status guidelines.
• I have added a brief description of why this PR is necessary and/or what this PR solves.
• Optional. My organization is added to USERS.md.
• Optional. For bug fixes, I've indicated what older releases this fix should be cherry-picked into (this may or may not happen depending on risk/complexity).

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