Build: allow opam 2.1.x

Opam 2.1.0 has been out for a few months, it's the default binaries to download from the website, or the default in Nixpkgs.

Manually testing the MR

Install Opam versions, and use the Makefile (the version check is global, for all uses of make).

Notes

Who is using opam calls …

 $ find src/ -name '*.sh' -exec grep opam {} \; | wc -l
0
 $ { find scripts/ | grep opam ;
     git grep opam scripts | cut -d: -f 1 ; } | sort -u | sed 's/.*/- [ ] `\0`/'

gives:

• scripts/ci/create_docker_image.build.sh, does not use opam just the repository
• scripts/ci/create_docker_image.minimal.sh, same
• scripts/ci/opam_handle_output.sh, does not call opam, it is meant for the CI, and depends on $HOME/.opam (broken for anybody overriding $OPAMROOT anyway).
• scripts/create_docker_image.sh, only opam-repo
• scripts/detect-opam-changes.sh, only looks at *.opam files
• scripts/env.sh, only tweaks CAML_LD_LIBRARY_PATH within its scope
• scripts/generate_opam_pipeline.sh generates opam-ci.yml
• scripts/install_build_deps.raw.sh: 2.1-updated
• scripts/install_build_deps.sh: 2.1-updated
• scripts/install_sapling_parameters.sh: works
• scripts/link_protocol.sh: does not call opam, generates .opam files
• scripts/opam-check.sh: only fails while running ./scripts/update_opam_repo.sh
• scripts/opam-pin.sh: works
• scripts/opam-remove.sh: works
• scripts/opam-test-all.sh: works
• scripts/opam-unpin.sh: works
• scripts/opam-upgrade.sh: works
• scripts/snapshot_alpha.sh: only tweaks .opam files
• scripts/update_opam_repo.sh: 2.1-updated
• scripts/version.sh: 2.1-updated

Checklist

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