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/default.nix

@@ -8,5 +8,6 @@
     ./nginx.nix
     ./opensearch.nix
     ./redis.nix
+    ./sops.nix
   ];
 }