Nightly E2E tests added for VM expression engine

The n8n CI pipeline now includes dedicated nightly tests for its VM-based expression engine. A new GitHub Actions workflow executes end-to-end tests daily at 04:00 UTC against the master branch, using the VM expression engine rather than the default. When these scheduled runs fail, a notification posts to the #project-secure-expression-evaluation Slack channel so the team can respond quickly. The workflow reuses existing CI infrastructure and can also be triggered manually for on-demand testing against any branch.

This addition targets the expression engine evaluation pipeline, ensuring the VM-based implementation receives continuous coverage as the codebase evolves. The dedicated nightly cadence means expression-related regressions surface within hours rather than waiting for weekly or release-based test suites.