Non-CI minimization should support minimizing against two versions of Coq using opam switches

[JasonGross]

Saying something like coqbot minimize coq.8.16 coq.8.17 (or coq.8.16 vs coq.8.17?) should have the minimizer set up two opam switches and two directory trees, run the script on both, determine automatically which one succeeds and which one fails, and then launch the minimizer against both builds