[ci] Test local switch installation

[ejgallego]

Dear opam reposotory devs,

we have found a few cases where opam packages are broken when:

• using a local switch [placed in _opam]
• the parent directory includes a dune-project file

in this case, dune may misdetect the root, and the package breaks. I'd say that this is a very specific problem of our package [coq] , however, it may be a good idea to test local opam switches in the CI.

[kit-ty-kate]

"transferred" (I can't transfer github issues accross different orgs apparently) to ocurrent/opam-repo-ci#137