Sporadic GitHub action run failures due to port unavailability

[fako1024]

In the E2E test we use an actual port to test distributed queries. Unfortunately this isn't always possible under the selected port on GitHub (all hail the cloud), as seen here.

DoD

• Improve flexibility (auto-select any available port)
• Fix racy global-query startup
• Stretch: Improve pipeline speed (concurrency)

[fako1024]

Happening again / still, e.g. here