Only run cargo build once when cargo libcnb package is run in monorepo root

[edmorley]

Currently if cargo libcnb package is run in the root of a CNB monorepo, cargo build is called multiple times - since it's run inside each buildpack directory, rather than being run once at the root of the repo.

As seen in eg:
https://github.com/heroku/buildpacks-jvm/actions/runs/5808991441/job/15746907911#step:9:14

This:
(a) causes Cargo to have to do extra work (since when building in single-crate mode, some things are repeated),
(b) likely means there is less efficient concurrent compilation, given that the Cargo job scheduler can't balance all crate compiles at once, but only groups of them in isolation.