chore(3p): Bump NixOS channels to 2021-02-18

Main motivation for this is to get the openldap update that fixes
10 CVEs: CVE-2020-36221 to including CVE-2020-36230. See also this
issue which lists them all: https://github.com/NixOS/nixpkgs/issues/113490

Someone should also redeploy whitby as soon as this lands in canon and
all build failures have been fixed.

Things done to resolve upstream breakages:

* grpc no longer takes abseil-cpp as an input, it has also been removed
  in the override.

* Upgrade glittershark's kernel to 5.11 since the linuxPackages_5_9
  attribute has been removed by upstream and the patch used by them is
  available for 5.11 as well.

* The fixed output hash for third_patry.apereo-cas changed for some reason.

* Remove the pin of haskellPackages.vector from the haskell overlay. It
  broke as the most recent version of vector in nixos-unstable no longer
  depends on semigroups. This effectively updates vector from 0.12.1.2
  to 0.12.2.0.

* Align two comments in tvix/libstore/worker-protocol.hh because the
  updated clang-format now demands that.

Change-Id: I2ecf10a98de935e9222acf1feaea447d4c11ed2d
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2538
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>
Reviewed-by: glittershark <grfn@gws.fyi>
Reviewed-by: sterni <sternenseemann@systemli.org>

third_party/nixpkgs-exposed/exposed/default.nix

@@ -79,7 +79,7 @@
     lib
     libredirect
     linuxPackages
-    linuxPackages_5_9
+    linuxPackages_5_11
     luajit
     lutris
     makeFontsConf