retry causes element to be undesirably reordered

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/290)
In GitLab by [[Gitlab user @abderrahimk]](https://gitlab.com/abderrahimk) on Mar 8, 2018, 13:41

When there is a build error, trying to hit r to retry: I find that more often than not, the build continues with other elements rather than retries.

I think it only works when the error is in the target I'm building (rather than its dependencies), but I'm not sure. I'm using `--builders 1` if that makes any difference.

UPDATE: From the comments below, and from recollection, the issue here is that retrying an element pushes the element back into the scheduler queues, but rather than pushing it to be the next task, it is appended to be executed later on, which is undesirable and confusing.

No file or test is named in the issue. Trace the `r` retry action into the scheduler queues, using `--builders 1` and a build error to observe ordering; done means the retried element becomes the next task rather than being appended behind other queued elements.

Written by the indexing model from the issue text.