Revert snap to amd64 only and update charm-ci

[javierdelapuente]

What this PR does

Revert snap to be only amd64. The charm is not adapted, and the publish fails, so no need for now.

Update charm-ci to fix the publish.

Why we need it

Checklist

• I followed the contributing guide
• I added or updated the documentation (if applicable)
• I updated docs/changelog.md with user-relevant changes
• I added a change artifact for user-relevant changes in docs/release-notes/artifacts. If no change artifact is necessary, I tagged the PR with the label no-release-note.
• I used AI to assist with preparing this PR
• I added or updated tests as needed (unit and integration)
• If integration test modules are used: I updated the workflow configuration
  (e.g., in .github/workflows/integration_tests.yaml, ensure the modules list is correct)
• If this PR involves a Grafana dashboard: I added a screenshot of the dashboard
• If this PR involves Terraform: terraform fmt passes and tflint reports no errors