yarn: set installation method to 'brew'

Fixes #10540
helps with https://github.com/yarnpkg/yarn/issues/2517

Closes #11343.

Signed-off-by: Tomasz Pajor <tomek@polishgeeks.com>

Formula/yarn.rb

@@ -19,6 +19,7 @@ class Yarn < Formula
   def install
     system "npm", "install", *Language::Node.std_npm_install_args(libexec)
     bin.install_symlink Dir["#{libexec}/bin/*"]
+    inreplace "#{libexec}/lib/node_modules/yarn/package.json", '"installationMethod": "tar"', '"installationMethod": "brew"'
   end
 
   test do