opam: Share opam root directory even on multiple instances
That avoid having multiple distinct opam root directories per opam lister instance, hence use less side effects (less disk space, less network) . The current opam commands used by the lister are actually listing specifically per instance.
Related to swh/meta$1163 Related to T3590
Test Plan
tox
Migrated from D6316 (view on Phabricator)