(on workers for opam lister and loader services)
This shall reduce the disk use to one opam root per worker (instead of worst case
scenario of x distincts opam root * y distincts instances, with x the number of workers,
y the unknown number of distinct opam instances).
Using /tmp/opam as default sounds sensible enough (providing we make sure
that it's initialized at boot time).
(then other diffs are required on the lister/loader for this to actually work   .
They are adaptations for the lister and loader to actually use correctcly such opam root).
Related to T3590