An env var affects the build of coq

[ana-borges]

If COQ_USE_DUNE is set, then the opam installation of coq will try to use dune and end up not installing anything.

Steps to reproduce:

export COQ_USE_DUNE=true
opam switch create test 4.12.0
opam update
opam repo add coq-released https://coq.inria.fr/opam/released
opam install coq -vv # observe dune is being used and an error is produced

Cf. coq/coq#14469, coq/opam#1736, and the zulip discussion.

[avsm]

We do need a coq package maintainer to submit these fixes to the opam-repository by forcing the variable to be unset in the build rules.

[github-actions]

This issue has been open 90 days with no activity. Consequently, it is being marked with the "stale" label. What this means is that the issue will be automatically closed in 30 days unless more comments are added or the "stale" label is removed. If you come across this issue in the future, you may also find it helpful to visit our forum at https://discuss.ocaml.org where queries related to OCaml package management are very welcome.