ci: build the e2e virtualenv once per worker and re-enable test-vscode-e2e - #6098

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: tim-actions/dco@master, tim-actions/get-pr-commits@master. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/