tools: only sign release if promotion successful

Ref: https://github.com/nodejs/build/pull/1596

PR-URL: https://github.com/nodejs/node/pull/24669
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

tools/release.sh

@@ -202,7 +202,9 @@ for version in $versions; do
 
     ssh ${customsshkey} ${webuser}@${webhost} $promotecmd nodejs $version
 
-    sign $version
+    if [ $? -eq 0 ];then
+      sign $version
+    fi
 
     break
   done