chore(nixpkgs): Bump channels to 2021-05-25

* users/grfn/system/home/yeren: remove obsolete awscli2 overrides

* ops: make new isSystemUser || isNormalUser assertion happy

* users/grfn/system/system/mugwump: make buildkite agents system users

* users/tazjin/nixos/camden: set isSystemUser = true for git

* users/tazjin/emacs: Remove missing & broken packages

* third_party/openldap: remove, as the argon2 module is now enabled upstream

* third_party/gerrit_plugins: Pinned new unstable hashes

* third_party/nix, third_party/grpc: Disabled CI as these are broken

* third_party/overlays/emacs: Bumped version to stay in sync with channel

* third_party/buzz: Update LIBCLANG_PATH to reference libclang.lib,
  since libclang's default output no longer contains libclang.so

* users/grfn/system/home: Install julia-stable instead of julia (which
  aliases to julia-lts), as the latter depends on an insecure version of
  libgit

Change-Id: Iff33b0ecb0ef07a82d1de35e23c40d2f4bf0f8ed
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3001
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>
Reviewed-by: grfn <grfn@gws.fyi>

users/tazjin/emacs/default.nix

@@ -33,7 +33,7 @@ let
   (with epkgs.melpaPackages; [
     ace-window
     ace-link
-    bazel-mode
+    # bazel-mode TODO(tazjin): where did this go?
     browse-kill-ring
     cargo
     company
@@ -47,7 +47,7 @@ let
     eglot
     elixir-mode
     elm-mode
-    erlang
+    # erlang
     go-mode
     gruber-darker-theme
     haskell-mode