Nix package via nix flakes

Added

• Nix flake for compilation and development environments

Fixed

• Tests in the crate::path::clean module now pass on Linux systems that don't have /usr/ as a top-level directory
  • This was discovered when trying to build a Nix package

Changes: v0.2.5...v0.2.6
Full Changelog: https://github.com/jrhawley/pad-path/CHANGELOG.md