Various minor fixes

lib/fudo/sites.nix

@@ -124,6 +124,7 @@ let
           my-build-host = {
             port = 22;
             systems = [ "i686-linux" "x86_64-linux" ];
+            build-user = "my-builder";
           };
         };
       };
@@ -194,6 +195,12 @@ let
         description = "List of features supported by this server.";
         default = [ ];
       };
+
+      build-user = mkOption {
+        type = str;
+        description = "User as which to run distributed builds.";
+        default = "site-builder";
+      };
     };
   };