chore(ci): Build OxCaml development branch

[Sudha247]

Fixes #13895.

Adds a workflow to test the build of OxCaml development branch, to catch any issues like #13889 early. Uses nix for faster builds. The workflow itself is more generic, such that any other critical rev deps can be added to the same workflow to test development builds.

Currently runs on pull_request for testing. We may only need to run this as part of release readiness later on.

[Leonidas-from-XIV]

I don't have a lot of experience with Nix so I'd rather suggest @Alizter takes a look at the part but here's some improvements for the Github action.

[Copilot]

Pull request overview

This PR adds CI coverage for building the OxCaml development branch against Dune’s development version, aiming to catch regressions earlier (per #13895 / #13889) by leveraging Nix for a more reproducible and faster setup.

Changes:

• Add a new GitHub Actions workflow to build OxCaml trunk on pull_request and workflow_dispatch.
• Extend the Nix flake with an ox-trunk devShell tailored for building OxCaml trunk (including a pinned Menhir source).
• Update flake.lock to include the new menhir-src flake input.

Reviewed changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated 2 comments.

File	Description
flake.nix	Adds menhir-src input and an ox-trunk devShell to support OxCaml trunk builds in CI.
flake.lock	Locks the new menhir-src input for reproducible builds.
.github/workflows/revdeps-dev-build.yml	Introduces a workflow that builds Dune from the PR, then builds OxCaml trunk using the new Nix devShell.

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[Leonidas-from-XIV]

LGTM

[shonfeder]

Looks OK to me. I am a bit unsure how this will generalize to other revdeps, since I'd rather think they could be tested with opam or dune facilities (e.g., via opam pin --dev-repo, but this looks like it should give us what we need to the oxcaml builds we need! Thanks :)

[Alizter]

FYI I have made some changes with how we run oxcaml packages in the main flake output here: #13999. Introduced there is a general mechanism that reads an arbitrary opam repository and translates it into nix derivations using the same sources and patches etc.

The motivation there was to use the oxcaml repository as the source of truth for oxcaml patches so that we could finally start using things like ppx_expect with oxcaml.

This should be the package set that you use for revdeps of oxcaml if that is what you want to test. However I should note that the opam repository patches won't cover oxcaml main changes.

[Sudha247]

The CI run is validated here: https://github.com/ocaml/dune/actions/runs/24086266630/job/70259907780?pr=13981. It takes about ~15 minutes. I've now changed it to just run on workflow_dispatch, to be used as a part of pre-release checks. This can be changed later to add more conditions to run it. I'll merge when the CI is green, unless there are any objections.

[Leonidas-from-XIV]

@Sudha247 have you considered running it as part of release branches automatically? It think this is a decent compromise that avoids burdening regular PRs with lengthy CI runs but does not require manual steps like workflow_dispatch does?

  push:
    branches:
      - '*-rc'

[Sudha247]

Good point! Done.