nixpkgs: bump

Fixes b/3.

Change-Id: I2f734422cdad00f78956477815c4aea645c6c49e
diff --git a/default.nix b/default.nix
index cc761d4..4540157 100644
--- a/default.nix
+++ b/default.nix
@@ -7,11 +7,11 @@
 
   readTree = import ./nix/readtree.nix {};
 
-  # Tracking nixos-unstable as of 2020-08-22.
-  nixpkgsCommit = "c59ea8b8a0e7f927e7291c14ea6cd1bd3a16ff38";
+  # Tracking nixos-unstable as of 2021-01-31.
+  nixpkgsCommit = "44ad80ab1036c5cc83ada4bfa451dac9939f2a10";
   nixpkgsSrc = fetchTarball {
-    url = "https://github.com/NixOS/nixpkgs-channels/archive/${nixpkgsCommit}.tar.gz";
-    sha256 = "1ak7jqx94fjhc68xh1lh35kh3w3ndbadprrb762qgvcfb8351x8v";
+    url = "https://github.com/NixOS/nixpkgs/archive/${nixpkgsCommit}.tar.gz";
+    sha256 = "1b61nzvy0d46cspy07szkc0rggacxiqg9v1py27pkqpj7rvawfsk";
   };
   nixpkgs = import nixpkgsSrc {
     config.allowUnfree = true;