Add a hook to disable Git download and use Git distributed with OS

Would it be possible to add a hook to `dugite` to disable any attempt at downloading a Git distribution at installation time and instead use the `git` binaries distributed with the system?

My need for this is twofold:

1. I'm using `dugite` (actually [surf-build](https://github.com/surf-build/surf) which depends on it in the latest betas) on platforms for which you don't distribute native binaries and probably never will (various BSDs).
2. Even on platforms where you do distribute native binaries, I'd prefer to use the system distribution rather than some random binary from the Internet.

I'm aware of `LOCAL_GIT_DIRECTORY` and `GIT_EXEC_PATH`, however those variables only have an effect at execution time, not at `npm install` time.

FWIW, you should be able to easily script detecting if Git is already correctly installed by attempting to run e.g. `git --version` and if successful using the output of `git --exec-path`.

Start by tracing the npm install path that currently downloads a Git distribution, then review the existing LOCAL_GIT_DIRECTORY and GIT_EXEC_PATH behavior. Check whether git --version succeeds and what git --exec-path returns; done means an installation hook can skip downloading and use the system Git across the stated platforms.

Written by the indexing model from the issue text.