Update nixpkgs.

nixpkgs-2105.json

-{
-  "name": "release2105",
-  "url": "https://releases.nixos.org/nixos/21.05/nixos-21.05.3916.3b1789322fc/nixexprs.tar.xz",
-  "sha256": "16czbb0kifzfxhg9g89d06csnfand7hjw829xbylbggmj9b8givq"
-}
+{
+  "name": "release2105",
+  "url": "https://releases.nixos.org/nixos/21.05/nixos-21.05.3980.f0869b1a2c0/nixexprs.tar.xz",
+  "sha256": "0bcwqc01qmvzf4jllik11cvpl5x8nwk83lwh1ic7bzwr95k1iifm"
+}