[nix] Upgrade to latest version of nixpkgs-unstable

Summary: This updates to the latest version of nixpkgs-unstable. This is in response to the need to update localstack from 1.4.0 to >2.3.2.

Test Plan: Successfully ran nix develop with working build environment

Reviewers: ashoat, varun, bartek, atul, O1 Nix

Reviewed By: ashoat, bartek, atul

Subscribers: atul, ashoat, tomek

Differential Revision: https://phab.comm.dev/D10561