chore(3p): Bump NixOS channels to 2020-12-28

Changes:

* ops/nixos/tvl-slapd: The NixOS module for OpenLDAP has removed the
  ability to configure OpenLDAP directly and now forces users to use
  some kind of weird Nix->OLC mapping that is mostly undocumented.

  This moves the config we need to the new format in a way that may or
  may not work and does the other arbitrary dance steps that someone
  decided to impose on us. Note that this now throws lots of warnings,
  but I can't be bothered to fix them.

* 3p: Random package removals accomodated

* users/glittershark: Pin grfn's kernel to 5.9, because the CK patch
  is not yet updated for 5.10

* users/glittershark: Update vendor hash for pg-dump-upsert, I suspect
  this changed because of something in the Go build machinery in
  nixpkgs. The deleteVendor flag also has no effect anymore and has been
  removed.

* users/glittershark: agda build is broken, commenting out development
  home-manager environment until it can be fixed

* third_party/haskell_overlay: updating random needs upper boundarles
  of a few dependencies relaxed (curse them)

* third_party/gerrit_plugins: for some cursed reason the fixed-output
  hash of the gerrit owners plugin fetchgit changed, updated.
  Same for the checks plugin.

Change-Id: Ica37995fe8039d3ba80eab643867f98795c56734
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2295
Tested-by: BuildkiteCI
Reviewed-by: Profpatsch <mail@profpatsch.de>
Reviewed-by: glittershark <grfn@gws.fyi>
Reviewed-by: tazjin <mail@tazj.in>

third_party/nixpkgs-exposed.nix

@@ -71,6 +71,7 @@
     lib
     libredirect
     linuxPackages
+    linuxPackages_5_9
     luajit
     lutris
     makeFontsConf
@@ -158,7 +159,7 @@
     mercurial
     perl
     perlPackages
-    utillinuxMinimal;
+    ;
 
   haskellPackages = (nixpkgs.haskellPackages.override {
     overrides = (import ./haskell_overlay { pkgs = nixpkgs; });