shell.nix: Update to a newer nightly (#11028)

substrate/shell.nix

@@ -2,10 +2,10 @@ let
   mozillaOverlay =
     import (builtins.fetchGit {
       url = "https://github.com/mozilla/nixpkgs-mozilla.git";
-      rev = "4a07484cf0e49047f82d83fd119acffbad3b235f";
+      rev = "15b7a05f20aab51c4ffbefddb1b448e862dccb7d";
     });
   nixpkgs = import <nixpkgs> { overlays = [ mozillaOverlay ]; };
-  rust-nightly = with nixpkgs; ((rustChannelOf { date = "2021-09-10"; channel = "nightly"; }).rust.override {
+  rust-nightly = with nixpkgs; ((rustChannelOf { date = "2022-02-10"; channel = "nightly"; }).rust.override {
     extensions = [ "rust-src" ];
     targets = [ "wasm32-unknown-unknown" ];
   });