.github: ci.yml: move AWS auth to just before it's needed - #3018

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4, actions/setup-node@49933ea5288caeca8642d1e84afbd3f7d6820020, oven-sh/setup-bun@735343b667d3e6f658f44d0eca948eb6282f2b76. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/