ci(fargate-runner): move required role to runtime policies

[DavSanchez]

Move the required role to runtime policies, as per docs for task_custom_policies, this might have been added to the wrong property:

Task execution (launching) custom policies json. The rights provided here will be added to the role launching the Fargate task.
[...]
These rights are different from the ones the processes running inside of the Fargate task have. For instance, if the task needs to launch some EC2 instances, you'd need to give it ec2:* rights through the task_runtime_custom_policies variable.

[alvarocabanas]

LGTM