[package] refer to %{buildroot} instead of $root/ROOT

Summary:
The build root can be overridden outside of the spec file. We should always use `%{buildroot}` instead.

Also bump the package version up to 2.9.5.

Test Plan: `arc package --statedir /var/facebook/watchman` used to fail. Now it works.

Reviewers: wez

Reviewed By: wez

Differential Revision: https://phabricator.fb.com/D1193812

Task ID: 3811660