All the `opam exec --` prefixes in the Makefile makes it hard to use with dune developer preview

[gridbugs]

Users of the dune developer preview will likely be unable to run phony targets from the Makefile since each invocation of dune has a opam exec -- prefix. The dune developer preview is not installed with opam, but running opam exec -- dune will have the effect of running the dune currently visible to opam. Thus if users are mixing direct invocations of dune (such as dune pkg lock to generate a lockdir) with make ... commands, the two different versions of dune may conflict.

I would recommend to remove the opam exec -- prefixes from the Makefile, and then if users/dockerfiles do want to run a make ... command from an opam environment they can run opam exec -- make ... instead. This gives users the choice of whether they want to run dune from an opam environment or from some other environment such as the one that is installed by the dune developer preview.