chore(3p): bump NixOS channels to 2021-11-03

* //third_party/{abseil_cpp, grpc}: fix linker problem by passing the
  C++ standard to use explicitly. nixpkgs upstream stopped passing this
  to abseil between bumps and the way this problem manifests itself is
  linker errors, because… C++, I suppose.

Change-Id: I932ea70befee90984ae2e575dfc23f5c601cd289
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3769
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>
Reviewed-by: grfn <grfn@gws.fyi>

third_party/grpc/default.nix

@@ -8,5 +8,6 @@
 }).overrideAttrs(orig: rec {
   cmakeFlags = orig.cmakeFlags ++ [
     "-DCMAKE_CXX_STANDARD_REQUIRED=ON"
+    "-DCMAKE_CXX_STANDARD=17"
   ];
 })