feat(grfn/mugwump): Set up agenix

Start setting up agenix with secrets in //users/grfn/secrets for
mugwump, starting with my cloudflare API key which I use for the ddns
from my home apartment

Change-Id: Ida66cb91da3415357a512039d6c23402f0ae9388
Reviewed-on: https://cl.tvl.fyi/c/depot/+/4683
Reviewed-by: grfn <grfn@gws.fyi>
Autosubmit: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI

users/grfn/secrets/shell.nix

@@ -0,0 +1,8 @@
+let
+  depot = import ../../.. {};
+in
+depot.third_party.nixpkgs.mkShell {
+  buildInputs = [
+    depot.third_party.agenix.cli
+  ];
+}