fix: Specify dolmen as dev dependency

[bclement-ocp]

Otherwise opam seems to ignore the actual commit hash provided...

[bclement-ocp]

OK, I don't understand why the macos and windows build are failing, but I assume they will start working again once dolmen is actually released and I'd be in favor of merging and living with those build failures for now. We'll fix them before the release (once dolmen itself is released).

[Halbaroth]

We got the following errors on macOS:

[ERROR] Could not synchronize /Users/runner/work/alt-ergo/alt-ergo/_opam/.opam-switch/sources/dolmen_type from "git+https://github.com/Gbury/dolmen.git#a0f1bc66e7256fff1068ac0df525a2d23c1f3ea7":
        "/opt/homebrew/bin/git fetch -q" exited with code 255 "error: cannot lock ref 'refs/tags/v0.4': 
Unable to create '/Users/runner/work/alt-ergo/alt-ergo/_opam/.opam-switch/sources/dolmen_type/.git/refs/tags/v0.4.lock': File exists."
[ERROR] Could not synchronize /Users/runner/work/alt-ergo/alt-ergo/_opam/.opam-switch/sources/dolmen_loop from "git+https://github.com/Gbury/dolmen.git#a0f1bc66e7256fff1068ac0df525a2d23c1f3ea7":
        "/opt/homebrew/bin/git fetch -q" exited with code 255 "error: cannot lock ref 'refs/tags/v0.4': 
Unable to create '/Users/runner/work/alt-ergo/alt-ergo/_opam/.opam-switch/sources/dolmen_loop/.git/refs/tags/v0.4.lock': File exists."
[ERROR] Could not synchronize /Users/runner/work/alt-ergo/alt-ergo/_opam/.opam-switch/sources/dolmen from "git+https://github.com/Gbury/dolmen.git#a0f1bc66e7256fff1068ac0df525a2d23c1f3ea7":
Could not retrieve some package sources, they will not be pinned nor installed:
        "/opt/homebrew/bin/git fetch -q" exited with code 255 "error: cannot lock ref 'refs/tags/v0.4':
Unable to create '/Users/runner/work/alt-ergo/alt-ergo/_opam/.opam-switch/sources/dolmen/.git/refs/tags/v0.4.lock': File exists."

It might be a bug with opam. Opam reported that it cannot retrieve these package sources and they will not
be pinned nor installed. Then, it installed them from the opam repository!