Pre pop cargo home cache (#512)

* debug (CI): putting reports in artifacts [skip ci]

* debug (CI): remove unneded jobs, revert lated

* change (doc): fix artifacts

* change (CI): CARGO_HOME goes to branch level to avoid collapses; pre-population for it

* change (CI): small fix to codecov

* change (CI): DRY prepop script

* change (CI): DRY prepop script

* change (CI): docstring for pre-pop cache script

* change (CI): no cache for codecov report
0 jobs for master