opam: Initialize opam root directory outside the constructor

If it's required at all, this will use the network to fetch and install it. This should be done outside the constructor.

Related to T3590

Test Plan

tox

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Migrated from D6317 (view on Phabricator)

Some references in the commit message have been migrated:

• T3590 is now swh/meta#3590 (closed)

Build is green

Patch application report for D6317 (id=22951)

Rebasing onto fabd950d...

Current branch diff-target is up to date.

Changes applied before test

commit f4556e0113fae658555b0c6f7296ac98c310c971
Author: Antoine R. Dumont (@ardumont) <ardumont@softwareheritage.org>
Date:   Tue Sep 21 19:15:31 2021 +0200

    opam: Initialize opam root directory outside the constructor
    
    If it's required at all, this will use the network to fetch and install it.
    This should be done outside the constructor.
    
    Related to [T3590](https://forge.softwareheritage.org/T3590 'view original for T3590 on Phabricator')

See https://jenkins.softwareheritage.org/job/DLDBASE/job/tests-on-diff/560/ for more details.

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