Tweak Makefile to allow not depending on `opam`

Context

When using a Nix Shell/Flake to build, test and so on, opam is not used and therefore not in scope. It would be nice to disable usage of opam conditionally so the Makefile can still be used.

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR