chore: Keep updating non-preferred gems if they've already been generated (googleapis#20596)

Author: dazuma

.toys/generate-updates.rb

@@ -73,12 +73,19 @@ def list_apis_versions
                         path: "discoveries/index.json", update: true)
   apis_versions = []
   JSON.parse(File.read path)["items"].each do |item|
-    next unless item["preferred"]
+    next unless item["preferred"] || gem_exists?(item)
     apis_versions << [item["name"], item["version"]]
   end
   apis_versions.shuffle
 end
 
+def gem_exists? item
+  name = item["name"]
+  version = item["version"]
+  gem_name = "google-apis-#{name}_#{version}"
+  File.file? "#{context_directory}/generated/#{gem_name}/#{gem_name}.gemspec"
+end
+
 def pr_single_gem api, version, index, total
   branch_name = "gen/#{api}-#{version}-#{@timestamp}"
   commit_message = "feat: Automated regeneration of #{api} #{version} client"