dev/ci: trial new agent dispatcher, add CI feature flags to env
Created by: bobheadxi
Part of https://github.com/sourcegraph/infrastructure/pull/3182 - switches the 10% rollout here to target the new dispatcher to test its scaling behaviour.
Closes https://github.com/sourcegraph/sourcegraph/issues/32843
Test plan
CI should pass in a reasonable amount of time. Spamming with main-dry-run as well to simulate load: https://buildkite.com/sourcegraph/sourcegraph/builds?branch=main-dry-run%2Fstateless-v2