Fixed/updated broken match blocks

I never set `programs.ssh.enable` so the match blocks weren't taking
effect (and were incorrectly set anyway).

users/alejandro/gpg.nix

@@ -16,13 +16,14 @@
     };
   };
 
-  programs.ssh.matchBlocks = [
-    {
-      "github.com" = {
-        publicKey = "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIAbpuKWQFiYhNfKXcYpjWFXsYMh7pyhXdGyV8JPBFzgl";
+  programs.ssh = {
+    enable = true;
+    matchBlocks = {
+      "pi" = {
+        user = "nixos";
       };
-    }
-  ];
+    };
+  };
 
   services.gpg-agent = {
     enable = true;