Skip npm publish if already published

This helps make the publish workflow idempotent.

ci/steps/publish-npm.sh

@@ -5,6 +5,14 @@ main() {
   cd "$(dirname "$0")/../.."
   source ./ci/lib.sh
 
+  # npm view won't exit with non-zero so we have to check the output.
+  local hasVersion
+  hasVersion=$(npm view "code-server@$VERSION" version)
+  if [[ $hasVersion == "$VERSION" ]]; then
+    echo "$VERSION is already published"
+    return
+  fi
+
   if [[ ${CI-} ]]; then
     echo "//registry.npmjs.org/:_authToken=${NPM_TOKEN}" > ~/.npmrc
   fi