Introduce sops and sops-nix for secret management

Use the GPG keys used for the password-store noc directory for the admin
keys.
Switch the git hosts secret management from colmena to sops-nix.

https://github.com/getsops/sops
https://github.com/Mic92/sops-nix

config/hosts/git/sops.nix

@@ -0,0 +1,7 @@
+{ ... }:
+
+{
+  sops = {
+    defaultSopsFile = ./secrets.yaml;
+  };
+}