Wait for GHA eval results instead of falling back to local evaluation

[GaetanLepage]

The default evaluation mode (github) attempts at fetching the evaluation results from the GitHub actions result.
Those will only be available a few minutes after the PR has been opened as they are not trivial to compute.

Currently, if the results are not available right away, nixpkgs-review falls back to local evaluation.
This PR changes this behavior. Now it will keep re-trying until the results are available.

Note: It is still possible to force local evaluation by using --eval local.

Motivation: Local evaluation is very resource-intensive (especially RAM). It should be an opt-in behavior.
This new approach is more predictable.

Questions:

• Does this work (at all) ?
• Is 10s a good value for the timeout ? -> Seems good, yes.
• Should we add a maximum amount of retries ?
  -> I have set a maximum waiting time of 10min:

• Handle Ctrl-C correctly (i.e. properly delete the worktree on exit)
  -> There is no worktree at this point. Just quit gracefully.

[GaetanLepage]

Maybe we should silence the warning:

[GaetanLepage]

This implementation is quite naive. We could eventually wait within the function and thus not restarting the fetching process from scratch each time.

[GaetanLepage]

Maybe we should silence the warning:

I removed them all. Do you have a better idea ?

[GaetanLepage]

I confirm that printing is fixed:

[GaetanLepage]

Should we add a tip:
"Use --eval local if you prefer running the evaluation locally."
?