Don't delete required artifacts when tracking is enabled

Tristan Van Berkomrequested to merge
tristan/fix-required-artifacts into master

Currently artifacts which are required for the current build session are deleted when dynamic tracking is enabled.

For instance, running bst build --track-all target.bst, will delete artifacts that are dependencies of target.bst in a cleanup job if the entire pipeline does not fit inside of the configured cache quota.

Merge request reports