Allow specifying pin file location when running @repo//:pin

We have multiple disparate environments (Android, YARN, "plain" jvm) that have disjoint dependencies, and we'd like to separate them all out. It's easy to do, but slightly clunky, as you have to pin and then move the file to be closer to the code and avoid cluttering up the repo root.

The pinning actually works fine if you move the file, and will correctly update the new file when you change the dependencies. The first run, however, puts the file at the repository root.

Start at the @repo//:pin entry point and trace where its first run writes the pin file at the repository root. Check how later runs update a moved file; done means allowing a specified location while preserving correct pin updates for separate environments.

Written by the indexing model from the issue text.