feat: add '--ssh' flag to update-ref tool to choose ssh url or default to https url

inventory.nix

@@ -120,12 +120,14 @@
           };
         };
       };
+    } // (if builtins.getEnv "ENABLE_USDA_DASH" == "1" then 
+    {
       "usda-dash" = builtins.fetchGit {
         url = "git@factory.uga.edu:MODEL/usda-dash-config.git";
         rev = "dab32f5884895cead0fae28cb7d88d17951d0c12";
         submodules = true;
       };
-    };
+    } else {});
     overrides = {
       athenix.host.useHostPrefix = false;
     };