chore(3p): Bump nixos-unstable channel

This bumps the channel to a commit that includes fixes for an nginx
issue we have been seeing:

https://github.com/NixOS/nixpkgs/pull/95264

Includes the following compatibility fixes:

- tests disabled in third_party.bufbuild: These were enabled
  unexpectedly by the update, but don't run in the sandbox because
  they want to download things from github

Change-Id: I98a3b5de57f62f1fd3a37701fa1896eddeedff85
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1759
Tested-by: BuildkiteCI
Reviewed-by: glittershark <grfn@gws.fyi>
Reviewed-by: tazjin <mail@tazj.in>

third_party/default.nix

@@ -5,11 +5,11 @@
 { ... }:
 
 let
-  # Tracking nixos-unstable as of 2020-08-04.
-  nixpkgsCommit = "840c782d507d60aaa49aa9e3f6d0b0e780912742";
+  # Tracking nixos-unstable as of 2020-08-16.
+  nixpkgsCommit = "16fc531784ac226fb268cc59ad573d2746c109c1";
   nixpkgsSrc = fetchTarball {
     url = "https://github.com/NixOS/nixpkgs-channels/archive/${nixpkgsCommit}.tar.gz";
-    sha256 = "14q3kvnmgz19pgwyq52gxx0cs90ddf24pnplmq33pdddbb6c51zn";
+    sha256 = "0qw1jpdfih9y0dycslapzfp8bl4z7vfg9c7qz176wghwybm4sx0a";
   };
   nixpkgs = import nixpkgsSrc {
     config.allowUnfree = true;