[Nix] Add nix installation script

Summary:
Add nix installation script

Currently this will:

• Install nix, if missing
• Prompt user for powerline installation
• Configure nix with:
  • flakes
  • trusted users (for cache)
  • Decent job and core defaults

This is implemented with a "reconcilation" configuration model, meaning
the individual actions should be idempotent. Thus safe to run many times.

Depends on D4843

Test Plan:

./scripts/install_nix.sh

# If missing nix, ensure that nix then exits (E.g. which nix)
# Try powerline prompt
# Assert nix.conf has max-jobs, cores, trusted-users, and flakes enabled

Reviewers: atul, abosh

Reviewed By: abosh

Subscribers: ashoat, tomek, adrian

Differential Revision: https://phab.comm.dev/D4844