Call the artifact CLI from nargo execute

[aakoshh]

Redirect nargo execute to noir-artifact execute, șo we get integration test coverage and remove the duplicated code between them.

Need to figure out how this "shelling out" should work exactly: perhaps the noir_artifact_cli crate can be built from the build.rs of nargo_cli, but should we assume that it's installed with noirup.sh for example, on CI, etc? Or maybe just turn more things into library functions.