github_packages: create hard link instead of copying

Some bottles are quite large, and the copies can make us run out of
space rather quickly. Let's try to avoid that by using hard links
instead of copies.

Library/Homebrew/github_packages.rb

@@ -458,7 +458,7 @@ def write_image_layout(root)
   def write_tar_gz(local_file, blobs)
     tar_gz_sha256 = Digest::SHA256.file(local_file)
                                   .hexdigest
-    FileUtils.cp local_file, blobs/tar_gz_sha256
+    FileUtils.ln local_file, blobs/tar_gz_sha256
     tar_gz_sha256
   end