Pin nixpkgs in tests

I've had enough with nixpkgs breakage affecting our tests...

.github/workflows/ci.yml

@@ -9,8 +9,6 @@ on:
 jobs:
   nixpkgs-fmt:
     runs-on: ubuntu-latest
-    env:
-      NIX_PATH: "nixpkgs=https://github.com/NixOS/nixpkgs/archive/nixos-unstable.tar.gz"
     steps:
     - uses: cachix/install-nix-action@v12
     - uses: actions/checkout@v1
@@ -22,10 +20,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest]
-        nixpkgs_version: ["master"]
         # os: [ubuntu-latest, macos-latest]
-    env:
-      NIX_PATH: "nixpkgs=https://github.com/NixOS/nixpkgs/archive/nixos-unstable.tar.gz"
     steps:
     - uses: cachix/install-nix-action@v12
     - uses: actions/checkout@v1

tests/default.nix

@@ -1,4 +1,12 @@
-{ pkgs ? import <nixpkgs> { } }:
+let
+  sources = import ../nix/sources.nix;
+in
+{ pkgs ? import sources.nixpkgs {
+    overlays = [
+      (import ../overlay.nix)
+    ];
+  }
+}:
 let
   poetry = pkgs.callPackage ../pkgs/poetry { python = pkgs.python3; inherit poetry2nix; };
   poetry2nix = import ./.. { inherit pkgs;inherit poetry; };