Freqent solver failures on packages

[shonfeder]

See

• https://docs.ci.ocaml.org/package/file-rewriter.0.0.3
• https://docs.ci.ocaml.org/package/camlkit-base.0.3.0
• Solver failure on learn-ocaml.0.14.1 #110

Reported at

• [new release] file-rewriter (2 packages) (0.0.3) ocaml/opam-repository#26650 (comment)
• [new release] Camlkit v0.3.0 ocaml/opam-repository#26837 (comment)

These packages build fine on opam ci, so this seems clearly like a problem with the bespoke solver used by the docs-ci.

Using a solver service could help. Is it possible we could just use opam with 0install as we do in opam-repo-ci? I assume there is a reason why not, but I don't have a clear sense of that reason.