CI is broken due to benchmarks dependency on core_bench

[mooreryan]

The current github action is broken due to the core_bench dependency in the benchmarks.

Example: https://github.com/ocaml/ocaml-re/actions/runs/3736175311/jobs/6413859261

Run opam exec -- dune build
File "benchmarks/dune", line 2, characters 23-33:
2 |  (libraries re threads core_bench)
                           ^^^^^^^^^^
Error: Library "core_bench" not found.
-> required by _build/default/benchmarks/benchmark.exe
-> required by alias benchmarks/all
-> required by alias default
Error: Process completed with exit code 1.

I was thinking of two possible fixes.

1. Add - run: opam install core_bench.v0.14.0 to the workflow yaml file before the dune build command is run. It has to be v0.14 for now as the benchmarks use some modules that were deprecated in v0.15. Main problem with this is that it's a heavy dep that will add some time to the CI.
2. Change opam exec -- dune build to dune build deprecated/ lib/ lib_test/ to avoid building the benchmarks in the CI. This seems like a good approach as the benchmarks are not run in the CI.

I have tested option 2 on the GitHub actions of my fork and it works fine for 4.14 action. (the 4.03 is still broken, but for reasons unrelated to this issue).

If one of these options sounds good to you, I can open a pull request for it.

[rgrinberg]

2. seems like a good approach