* Fix the output of the nixpkgs derivation
* Use the fixed-output sha256 hash for builtins.fetchTarball
This change makes the nixpkgs pin forwards compatible with Nix 1.12.x
and Nix 2.0.
* Correct the phrasing of the code comment
This is a "shallow" update, meaning that there are still several dependencies
(such as `grpc` or Haskell packages) that are still pinned and those pins might
not be necessary any longer after this change.
* Pin nixpkgs to `7ae9da426924537755ce9164fd5b5f81ce16a1c3`; minor documentation tweaks
* Remove use of `fetchgitPrivate` from `proto3-suite` toplevel pkg override attr def
