ci: configure npm auth and pass GITHUB_TOKEN for unstable publish

[soberm]

Description of changes

changeset publish calls npm publish under the hood, and npm requires auth credentials in ~/.npmrc. Previously, changesets/action handled writing the npm token to ~/.npmrc automatically. When the switch to a direct run: step was made, that setup was lost — NPM_TOKEN was passed as an env var, but npm doesn't read auth from environment variables.

This PR adds a dedicated step to write the npm auth token to ~/.npmrc before publishing, matching the pattern used by the existing npm-publish composite action in this repo (.github/actions/npm-publish/action.yml).

Issue #, if available

Fixes the Release Unstable workflow ENEEDAUTH failure: https://github.com/aws-amplify/amplify-js/actions/runs/24179532870/job/70568727701

Description of how you validated changes

Checklist

• PR description included
• yarn test passes
• Unit Tests are changed or added
• Relevant documentation is changed or added (and PR referenced)

Checklist for repo maintainers

• Verify E2E tests for existing workflows are working as expected or add E2E tests for newly added workflows
• New source file paths included in this PR have been added to CODEOWNERS, if appropriate

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[changeset-bot]

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Latest commit: bd5cd8e

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