I’ll add, that since i’m not really trying to get on the leaderboard, I take a little more time to use these problems as an excuse to explore some topics/libraries from the ecosystem. Expect tests have been a very nice and light-weight way to write, ehm “expectations” while I work on intermediate steps during a problem (it could be testing my parser, testing the behavior of other data structures/processes i intend to use etc).
Where is your euler package coming from? 
Just fixed the dependencies so GitHub actions has the right URL now (https://github.com/bcc32/projecteuler-ocaml). I don’t know how to make dune-project generate a pinned dependency so that doesn’t work yet.
You can put the pin-depends field (in opam syntax) in a package.opam.template file next to package.opam. It will be copied verbatim when generating the opam file.
Thanks, that seems to have worked.