update_bench_status should check whether status is valid before querying GitHub.

[Zimmi48]

In update_bench_status, there are a few statuses ("created", "pending"...) that won't trigger any status check creation on GitHub:
https://github.com/coq/bot/blob/a6605542c2344a54859af39284c9149b388563d2/src/actions.ml#L505-L508

But we query GitHub in all cases, only to discard the result of the query in the cases mentioned above:
https://github.com/coq/bot/blob/a6605542c2344a54859af39284c9149b388563d2/src/actions.ml#L437

We should check that we are in one of the cases where a status check will be created on GitHub before doing anything else.