ci: cache on github actions and fix dune @fmt rule

[katrinafyi]

approach is following the idea of https://discuss.ocaml.org/t/cache-with-gh-action-ocaml-setup-ocaml-v2/11650/7. we also use composite actions to factor it out from each workflow file

cached build times are down to 1-2 minutes, down from 6+ minutes.

[katrinafyi]

i'm losing it. in ci, we have

[NOTE] Package ocamlformat is already installed (current version is 0.29.0).

i have the same one on my computer.

in ci, if we do opam exec -- dune build @fmt, it shows errors. on my computer, if we do that command, it shows no errors.

[JTrenerry]

Maybe it is time for

Warning: No ocamlformat version found in .ocamlformat file. It's recommended to specify the version in your .ocamlformat file for better consistency.

[agle]

I tried docker and its only marginally faster https://github.com/agle/bincaml/actions/runs/24381285654/job/71205114682