`evergreen_activate_gen_tasks.py` tried to raise the Evergreen API retry count by reassigning `evg_api._http_retry` after building the client, but that never took effect — the client mounts its HTTP adapter at construction, so the reassignment was ignored. Task generation has been running on default retries this whole time, and the POST that activates tasks was never retried at all.

`evergreen_activate_result_tasks.py` already worked around this by re-mounting the adapter. Extract that into `buildscripts/util/evergreen_retry.py` as `get_extra_retry_evergreen_api()` and call it from both scripts.

• No behavior change for `evergreen_activate_result_tasks.py`.
• Task generation now actually gets the extra attempts, and POST activation retries.