Oops, fix symlink for /public-data/latest.sql.gz

Oh whoops, I was symlinking to the *full* path of the latest dump,
which includes the site version directory in it. This meant that, if 5
new versions of the app were deployed since the most recently public
data commit (and so that version is deleted), the symlink fails.

In this change, we just symlink to the filename, which behaves as a
relative path and should be completely resilient to deploys changing
where these files ostensibly live!!

lib/tasks/public_data.rake

@@ -57,7 +57,7 @@ namespace :public_data do
 		# Link this latest dump as `latest.sql.gz`.
 		latest_path = Rails.configuration.public_data_root / "latest.sql.gz"
 		File.unlink(latest_path) if File.symlink?(latest_path)
-		File.symlink(dest_path, latest_path)
+		File.symlink(filename, latest_path)
 
 		puts "Linked dump to #{latest_path}"
 	end