fix `make build-dep`

This fixes make build-dep. It does so in a way that depends on opam 2.

I wonder if it is too early to depend on opam 2. Notice that the Coq "core-dev" repo has been converted to opam 2 in November already, but the repos containing user packages are still opam 1.2-compatible. On the other hand, I really have no desire to test this stuff with opam 1.^^