opam: retrieve opam from $PATH with shutil.which

The default behavior of subprocess is to pull executables from a hardcoded list, which doesn't work when opam is installed manually in the user's home directory.

changed milestone to %Design and test next generation CI Automation [Roadmap - Tooling and infrastructure]

added activity::MRO label

assigned to @olasd

Jenkins job DLDBASE/gitlab-builds #141 succeeded .
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merged