ocaml-ci is breaking builds that use pin depends

[shonfeder]

Context

This is showing up in CI jobs for projects that use vendored dependencies (via git submodules) as

...
[ERROR] The compilation of ocluster-api.dev failed at "dune subst".
...
#=== ERROR while compiling ocluster-api.dev ===================================#
# context     2.3.0 | linux/x86_64 | ocaml-base-compiler.5.2.1 | pinned(file:///src/ocluster)
# path        ~/.opam/5.2/.opam-switch/build/ocluster-api.dev
# command     ~/.opam/5.2/bin/dune subst
# exit-code   1
# env-file    ~/.opam/log/ocluster-api-1-306113.env
# output-file ~/.opam/log/ocluster-api-1-306113.out
### output ###
# File ".", line 1, characters 0-0:
# Error: There is no dune-project file in the current directory, please add one
# with a (name <name>) field in it.
# Hint: 'dune subst' must be executed from the root of the project.

I've seen it in these two builds, so far:

• https://ocaml.ci.dev/github/ocurrent/ocaml-ci/commit/cc9bac43c99dedc42a48b4dc1982157f87a98132/variant/alpine-3.20-5.2_opam-2.3
• https://github.com/ocurrent/ocurrent-deployer/runs/33726197377

Step(s) to reproduce

This is showing up in ocaml-ci running on itself, starting with cc9bac4

Expected behaviour

It should build vendored dependencies OK.

Technical details

Since I think the earliest this showed up is in the CI results run on cc9bac4 it's possible that some of the upstream changes pulled in from that change are responsible. But this is just a guess as I have not had time to investigate properly yet.