Check if there's a `pin-depends` field in .opam

[pitag-ha]

The pin-depends field has no effect on the package repository and, if I understand correctly, should be removed from the opam file before starting the release. When releasing Irmin, they perform that check manually: see here.

I suggest to add a lint check that checks if the opam file contains a pin-depends field. So the user would be warned in that case both when running dune-release check and when running dune-release distrib without the --skip-lint option.