Document situations where not pinning the nix version changes outputs

In https://github.com/nh2/static-haskell-nix/blob/d24dea3d46a727e1cd93e67458ce2768109efe0a/static-stack2nix-builder-example/default.nix#L36 I pin the version of `nix` itself.

I added that in the past because the upgrade from Nix 1.* to 2.0 created (if I remember correctly) different `.drv` files.

I would like to document cases beyond that which resulted in different `.drv`s or build outputs when the version of nix changes, so that I can justify to continue pinning `nix`:

## `.drv` changes

* During the ugprade from nix 1.* to 2.0

## Output / build changes

* Python unicode errors: https://github.com/NixOS/nixpkgs/pull/75676#issuecomment-569163012