dev/ci: bump stateless agent rollout to 50

Created by: bobheadxi

Give us a better view for comparing some recently flakey steps by splitting the new stateless and old stateful builds equally

The rollout was set to 100% accidentally (https://github.com/sourcegraph/sourcegraph/pull/33115), and I've adjusted the behaviour for the dispatcher a bit to keep up better - hopefully it's in a good shape to make this bump.

Test plan

See https://github.com/sourcegraph/infrastructure/pull/3182 for observability measures. We can easily disable the rollout by adjusting the line edited in this PR.