opam loader: Adapt for opam > 2.1

Current opam version > 2.1 (unstable) won't work as per the current debian build failures [1] [2].

opam show cli that the loader implementation relies on has changed in newest version.

• [1] https://jenkins.softwareheritage.org/job/debian/job/packages/job/DLDBASE/job/gbp-buildpackage/262/console (old bwrap issue no longer happening, kept for history purpose)

• [2] https://jenkins.softwareheritage.org/job/debian/job/packages/job/DLDBASE/job/gbp-buildpackage/263/console (no longer bwrap as tests no longer trigger actual opam calls)

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Migrated from T3976 (view on Phabricator)