nix: pin mozilla overlay

default.nix

@@ -1,9 +1,5 @@
-{
-  mozillaOverlay ? import (builtins.fetchTarball https://github.com/mozilla/nixpkgs-mozilla/archive/master.tar.gz),
-}:
-
 let
-  pkgs = import <nixpkgs> { overlays = [ mozillaOverlay ]; };
+  pkgs = import <nixpkgs> { overlays = [ (import ./mozilla-overlay.nix) ]; };
   rustPlatform = (import ./rustPlatform.nix { inherit pkgs; });
   artiqpkgs = import <artiq-fast/default.nix> { inherit pkgs; };
   vivado = import <artiq-fast/vivado.nix> { inherit pkgs; };

mozilla-overlay.nix

@@ -0,0 +1,10 @@
+let
+  pkgs = import <nixpkgs> {};
+  overlay = pkgs.fetchFromGitHub {
+    owner = "mozilla";
+    repo = "nixpkgs-mozilla";
+    rev = "efda5b357451dbb0431f983cca679ae3cd9b9829";
+    sha256 = "11wqrg86g3qva67vnk81ynvqyfj0zxk83cbrf0p9hsvxiwxs8469";
+  };
+in
+  import overlay

shell.nix

@@ -1,6 +1,5 @@
 let
-  mozillaOverlay = import (builtins.fetchTarball "https://github.com/mozilla/nixpkgs-mozilla/archive/master.tar.gz");
-  pkgs = import <nixpkgs> { overlays = [ mozillaOverlay ]; };
+  pkgs = import <nixpkgs> { overlays = [ (import ./mozilla-overlay.nix) ]; };
   artiq-fast = <artiq-fast>;
   rustPlatform = (import ./rustPlatform.nix { inherit pkgs; });
   artiqpkgs = import "${artiq-fast}/default.nix" { inherit pkgs; };