Git pre-push hook should be generated to location defined by `core.hooksPath`

In our project, we define all our git hooks at `.githooks`, so we can version control them and have them shared amongst our team. We recommend that those in our project set up the git hooks by running `git config --local unset core.hooksPath`.

The git pre-push hook generated by `./gradlew spotlessInstallGitPrePushHook` writes the hook to `.git/hooks`, instead of wherever `git config core.hooksPath` is pointing to. By default, `git config core.hooksPath` is equivalent to `.git/hooks`, but it can be customised to something else, like in our case.