chore(3p/nixpkgs): Bump channels to 2021-06-05

Includes the following depot changes & fixes:

* stable moves to NixOS 21.05
* stable isn't used anymore (but we'll keep the mechanism)
* haskell overlay's `random` override is removed (YAY!)
* grfn/iso: Switch to regular kernel rather than
  latest kernel, as latest kernel is currently marked as broken due to zfs
* grfn/home: Use julia_16-bin temporarily
  julia 1.5 (current julia-stable, source built release in nixpkgs)
  doesn't pass its own test suite. Julia 1.6 doesn't have a source built
  package in nixpkgs yet, so julia_16-bin appears to be the only working
  julia derivation currently.
* tazjin/tverskoy: Use zfs unstable, as stable zfs doesn't work with the
  latest kernel

Co-Authored-By: Griffin Smith <grfn@gws.fyi>
Co-Authored-By: sterni <sternenseemann@systemli.org>
Change-Id: I6f2e3d9f75077e4755de6bde9104d44b584cbe4c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3174
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>
Reviewed-by: tazjin <mail@tazj.in>
Reviewed-by: grfn <grfn@gws.fyi>

users/grfn/system/home/modules/development.nix

@@ -76,7 +76,9 @@ with lib;
 
     nodePackages.prettier
   ] ++ optionals (stdenv.isLinux) [
-    julia-stable
+    # TODO(grfn): replace with stable again once the current julia debacle
+    # is resolved upstream, see https://github.com/NixOS/nixpkgs/pull/121114
+    julia_16-bin
     valgrind
   ];