opam: Directly use the --root flag instead of using an env variable
This aligns the behavior with the opam loader.
@aleo it's actually a good idea that I did this slight adaptations ;). Now I recall that
for the deployment [1], it would work but often passing through the opamroot
initialization step in the loader. Because, there is no guarantee that the actual worker
listing the package (with a given opamroot as /tmp/<something>) will be the same one
ingesting the package. Thus, the opamroot passed along to the loader may not exist, so
it would get initialized and possibly multiple times.
We should probably make that opamroot a configuration entry in the dedicated configuration file targeting a fixed location in the workers to avoid the too many opamroot locations in /tmp (for another time/diff).
Related to #3358 (closed)
Migrated from D6010 (view on Phabricator)