Add CI job building irmin with dune package management

[ElectreAAS]

Hello irmin folks!
We've been wanting to make sure that irmin can be built using dune package management, and this PR introduces a CI job that can make that check automatically.
If you were to try building irmin with 'stock' dune (3.20.2) with pkg enabled, you'd hit a error related to irmin-watcher.
Since ocaml/dune#13232, this is now fixed, and dune (on its main branch) can be used by irmin devs as a daily driver!

Note that this PR is built on top of #2387

[zshipko]

Awesome, I am excited to use dune pkg to work on Irmin! I just tried it out using dune pkg lock && dune pkg runtest works as expected using this branch!

It's not clear what is going on with formatting when using opam packages but I will try to investigate

[art-w]

This PR looks good, thanks! However it's blocked by the weird (existing) CI failure, which we attempted to fix in #2387 without success

[zshipko]

This PR looks good, thanks! However it's blocked by the weird (existing) CI failure, which we attempted to fix in #2387 without success

CI is currently green without #2387 (for instance #2385) but it seems like there is a discrepancy between the formatting used when running the tests with dune and opam.

I can spend a moment trying to figure out what's happening here today.

[zshipko]

The issue is that dune pkg is using ppxlib.0.37.0 while opam is selecting ppxlib.0.35.0 and they have incompatible output.

Maybe we need to require ppxlib >= 0.37.0 in ppx_irmin.opam in #2387 instead?

[art-w]

Yeah we already tried this but then the CI doesn't find a solution for the dependencies ( https://ocaml.ci.dev/github/mirage/irmin/commit/ed85a176f6637df008bbf8f9d9e655746436286b/variant/%28analysis%29 )

[zshipko]

The problem seems to be bisect_ppx: 386c49f

[ElectreAAS]

Thanks Zach for merging this :)