Export solver-cache-03 entries as JSON

[tmcgilchrist]

Currently solver-cache entries are written as binary representations of the solve result.
This makes it difficult for humans to read them and debug the results of solves that fail.
With a minimal amount of work they could be written to disk as JSON and the solver-cache version bumped to v3.
See https://github.com/ocurrent/ocaml-docs-ci/blob/main/src/lib/solver.ml#L100-L134