Upload the artifact required by Searchfox and index code coverage task as soon as possible

[marco-c]

Currently we index the code coverage task and upload artifacts at the end of the task. Almost all our tasks take less than 190 minutes to finish.
This means there is a ~190 minutes delay between the time all m-c coverage tests finish and the time the coverage artifact is available to Searchfox.

If we uploaded the artifact as soon as it is generated and indexed the task right after that, we'd considerably reduce this delay.