Dynarray test PR broke scheduled 5.0 and 5.1 workflows

[jmid]

The Dynarray PR #463 predated our switch away from using opam in our workflows #473.

This unfortunately meant that the test for 5.2 (when Dynarray was added) - by using an opam variable in src/dynarray/dune - worked when it was implemented - but now no longer works on the 5.0 and 5.1 workflows as the looked up ocaml_version variable is non-existent.

This can be fixed, e.g., by generating a dune file, e.g., echoing "Dynarray tests disabled on <5.2.0" in the above case.