fix: install plugins over HTTPS so no SSH key is needed - #25

Process completed with exit code 1.

'flow' has no source.repo

'steuer' has no source.repo

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. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/