Exclude builders/ directory from gem package

`rake native gem` somehow fails due to symlinks under builders/ directory.

ruby_wasm.gemspec

@@ -22,7 +22,7 @@ Gem::Specification.new do |spec|
   spec.files = Dir.chdir(__dir__) do
     `git ls-files -z`.split("\x0").reject do |f|
       (f == __FILE__) || f.match(%r{\A(?:(?:bin|test|spec|features)/|\.(?:git|travis|circleci)|appveyor)}) ||
-        f.match(%r{\Apackages/})
+        f.match(%r{\A(?:packages|builders)/})
     end
   end
   spec.bindir = "exe"