build(deps): bump nixpkgs-unstable from 90c100c to 73c703c

Bumps [nixpkgs-unstable](https://github.com/NixOS/nixpkgs) from `90c100c` to `73c703c`.
- [Commits](NixOS/nixpkgs@90c100c...73c703c)

---
updated-dependencies:
- dependency-name: nixpkgs-unstable
  dependency-version: 73c703c22422b8951895a960959dbbaca7296492
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]