Remove invalid tag export from publish workflow

[Copilot]

Description

The failing publish-ghcr build job was aborting in Get tag before any build/sign steps ran. The workflow wrote a bare version string to $GITHUB_ENV, which GitHub rejects as an invalid env-file entry.

• Root cause

  • Get tag emitted a raw tag/version value into $GITHUB_ENV.
  • The current workflow does not consume TAG downstream.

• Change

  • Remove the redundant Get tag step from .github/workflows/publish-package.yml.
  • Leave version/reference derivation in prepare, which already provides the values used by later steps.

- name: Get tag
  run: |
    echo "TAG=${GITHUB_REF#refs/tags/}" >> $GITHUB_ENV

Which issue(s) does this PR resolve?

Type of change

• Bug fix (non-breaking change which fixes an issue)

Testing and verification

Validated the fix against the referenced Actions failure mode: this change removes the invalid env-file write that caused the job to fail before image build and signing.

Checklist

• Does the affected code have corresponding tests?
• Are the changes documented, not just with inline documentation, but also with conceptual documentation such as an overview of a new feature, or task-based documentation like a tutorial? Consider if this change should be announced on your project blog.
• Does this introduce breaking changes that would require an announcement or bumping the major version?
• Do all new files have appropriate license header?

Post merge requirements

• MAINTAINERS: manually trigger the "Publish Package" workflow after merging any PR that indicates Helm Chart Change