Downgrade sign tool

Downgrade the `sign` tool to the same version used when we published v8.4.1 to see if it's the source of the failure.

Author: martincostello

.config/dotnet-tools.json

@@ -33,10 +33,10 @@
       ]
     },
     "sign": {
-      "version": "0.9.1-beta.24406.1",
+      "version": "0.9.1-beta.24312.3",
       "commands": [
         "sign"
       ]
     }
   }
-}
+}