chore(3p): bump NixOS channels to 2021-03-18

Making this a monthly service apparently.

Necessary changes:

* 3p: expose emacs27 instead of emacs26 which got removed
  users/tazjin/{camden, frog}: switch from emacs26 to emacs27

* 3p/lieer: google_api_python_client got renamed to
  google-api-python-client

Change-Id: I1011665d10eebc99990addbef6a8a6b000b93896
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2605
Tested-by: BuildkiteCI
Reviewed-by: glittershark <grfn@gws.fyi>
Reviewed-by: tazjin <mail@tazj.in>

users/tazjin/nixos/camden/default.nix

@@ -142,7 +142,7 @@ in lib.fix(self: {
       bat
       curl
       direnv
-      emacs26-nox
+      emacs27-nox
       fswebcam
       git
       gnupg