Pin nixpkgs to a recent commit on the 20.03 channel

Motivation:

- Channel: Larger set of packages in the binary cache compared pinning
  an arbitrary commit.
- 20.03: Easier to have similar nixpkgs between projects. Release
  channels move much more slowly, so it is more likely that packages
  are shared between builds.

nix/sources.json

@@ -55,15 +55,15 @@
  "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
  },
  "nixpkgs": {
- "branch": "nixos-unstable",
+ "branch": "nixos-20.03",
  "description": "A read-only mirror of NixOS/nixpkgs tracking the released channels. Send issues and PRs to",
  "homepage": "https://github.com/NixOS/nixpkgs",
  "owner": "nixos",
  "repo": "nixpkgs-channels",
- "rev": "7c399a4ee080f33cc500a3fda33af6fccfd617bd",
- "sha256": "0vqljvz5yrc8i3nj3d5xiiv475yydscckfc9z0hpran9q2rh4md1",
+ "rev": "025deb80b2412e5b7e88ea1de631d1bd65af1840",
+ "sha256": "09mp6vqs0h71g27w004yrz1jxla31ihf18phw69wj61ix74ac4m0",
  "type": "tarball",
- "url": "https://github.com/nixos/nixpkgs-channels/archive/7c399a4ee080f33cc500a3fda33af6fccfd617bd.tar.gz",
+ "url": "https://github.com/nixos/nixpkgs-channels/archive/025deb80b2412e5b7e88ea1de631d1bd65af1840.tar.gz",
  "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
  },
  "xlm-roberta-base-sentencepiece": {

tests.nix

@@ -23,7 +23,7 @@ let
  sentencepiece-sys = attr: {
  nativeBuildInputs = [ pkgconfig ];
 
- buildInputs = [ (sentencepiece.override (attrs: { inherit stdenv; })) ];
+ buildInputs = [ sentencepiece ];
  };
 
  torch-sys = attr: {