@obazl//coswitch:refresh fails on local switch with system compiler

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Use case: system compiler is 4.14.0. To create a local switch in ppx_assert: opam switch create . 4.14.0. This creates a new local switch using the system compiler (/usr/local/bin/ocamlc etc.), which means that ./_opam/ocaml will not have all the stuff it would have if the compiler is installed in the switch itself. Which means in turn that @obazl//coswitch:refresh will not know how to deal with it.