chore(3p/nixpkgs): update channels to 2021-07-25

* 3p/buzz: bump to latest master (1.6.0)

* 3p/emacs/explain-pause-mode: adjust to package-build update
  MELPA's package build now cares about git revisions, but calling VC
  commands in a nix build is usually a bad idea. Thus upstream nixpkgs
  passes `$commit` to the `buildPhase` and otherwise fails with an
  error message that doesn't really point to the issue. Upstream change:
  9140d4b06f

* 3p/overlays/emacs: udpate to 2021-07-25 to support the package-build
  update. Without this emacsPackages.xelb (for tazjin's exwm) would fail.

Change-Id: I7cd782fe7d66ed4ea78c529b79fe761d921f46a8
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3253
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>
Reviewed-by: grfn <grfn@gws.fyi>

third_party/overlays/emacs.nix

@@ -2,10 +2,10 @@
 { ... }:
 
 let
-  # from 2020-05-26
-  commit = "5df3462dda05d8e44669cf374776274e1bc47d0a";
+  # from 2021-07-25
+  commit = "850ddaaadd4c640bd8eb4b154734a96bbe234d07";
   src = builtins.fetchTarball {
     url = "https://github.com/nix-community/emacs-overlay/archive/${commit}.tar.gz";
-    sha256 = "0ggmkg4shf9948wpwb0s40bjvwijvhv2wykrkayclvp419kbrfxq";
+    sha256 = "178ysnqs6k7y8h9zvd3l7c0m6lqzncn3dh0fb8sqv4hgsvx95j6r";
   };
 in import src