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

flake.lock

@@ -52,6 +52,22 @@
         "type": "github"
       }
     },
+    "nixpkgs-stable": {
+      "locked": {
+        "lastModified": 1716061101,
+        "narHash": "sha256-H0eCta7ahEgloGIwE/ihkyGstOGu+kQwAiHvwVoXaA0=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "e7cc61784ddf51c81487637b3031a6dd2d6673a2",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "release-23.11",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
     "nixpkgs-unstable": {
       "locked": {
         "lastModified": 1716479278,
@@ -72,7 +88,29 @@
       "inputs": {
         "nixos-generators": "nixos-generators",
         "nixpkgs": "nixpkgs",
-        "nixpkgs-unstable": "nixpkgs-unstable"
+        "nixpkgs-unstable": "nixpkgs-unstable",
+        "sops-nix": "sops-nix"
+      }
+    },
+    "sops-nix": {
+      "inputs": {
+        "nixpkgs": [
+          "nixpkgs"
+        ],
+        "nixpkgs-stable": "nixpkgs-stable"
+      },
+      "locked": {
+        "lastModified": 1716400300,
+        "narHash": "sha256-0lMkIk9h3AzOHs1dCL9RXvvN4PM8VBKb+cyGsqOKa4c=",
+        "owner": "Mic92",
+        "repo": "sops-nix",
+        "rev": "b549832718b8946e875c016a4785d204fcfc2e53",
+        "type": "github"
+      },
+      "original": {
+        "owner": "Mic92",
+        "repo": "sops-nix",
+        "type": "github"
       }
     }
   },