Integrate nix build into packer/ansible build

[samrose]

Chore

This will include:

• programmatic installation and configuration of nix on the ami using the open source https://github.com/DeterminateSystems/nix-installer
• Installation via nix profile install for the appropriate user, of the cached version of .#psql_15/bin (15.6 version)
• Attempting to minimize changes as much as possible along the way
• Thorough documentation of changes, and non-jargon explanation of what is happening