Introduce NixOS support (download proper kustomize binary)

[AleksanderGondek]

Related Issue

Implements feature request 54

Motivation and Context

Allow rules_gitops to work on NixOS without any additional patching.

How Has This Been Tested?

I have been running similar patch for quite time on my NixOS-based machines.

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] I have signed the Adobe Open Source CLA.
• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.