add retries around final scmake in loop_tasks

retries probably go right here: https://github.com/USGS-R/scipiper/blob/master/R/loop_tasks.R#L142

useful the 2nd time you run a multi-task job, when you already have the .ind files and may not want to delete them (at the accepted cost of waiting longer for remake to hash all the files each time) but want the retries too.

Start in R/loop_tasks.R at line 142, where the final scmake call is made. Reproduce a second multi-task job with existing .ind files and inspect the current behavior before deciding how retries should apply. Done means the final scmake step retries in that scenario without requiring the .ind files to be deleted.

Written by the indexing model from the issue text.