[Nix] Configure nix before attempting Powerline install

Summary:
The nix build and nix eval commands need
the experimental features enabled in the nix.conf before these can succeed.

Originally just ordered the events by my task order, but the ordering needs to
configure nix first on a true new nix install.

Depends on D4816

Test Plan:
From a machine which doesn't have nix on it:

./scripts/install_nix.sh
# validate nix and flakes are enabled
nix run nixpkgs#cowsay -- it works!

Or from a existing mac machine with nix on it:

# Revert nix.conf to its original install state
echo "build-users-group = nixbld" | sudo tee /etc/nix/nix.conf > /dev/null

# Run script
./scripts/install_nix.sh

# Check contents have changed
cat /etc/nix/nix.conf

Reviewers: abosh, ashoat, varun

Reviewed By: abosh

Subscribers: tomek, atul

Differential Revision: https://phab.comm.dev/D4862