api: use formulae.brew.sh for cask-source API again.

GitHub's raw endpoint is proving hilariously unreliable for us here.

Library/Homebrew/api.rb

@@ -100,8 +100,11 @@ def self.fetch_file_source(filepath, repo:, git_head: nil)
       endpoint = "#{git_head}/#{filepath}"
       return cache[endpoint] if cache.present? && cache.key?(endpoint)
 
-      raw_url = "https://raw.githubusercontent.com/#{repo}/#{endpoint}"
-      output = Utils::Curl.curl_output("--fail", raw_url)
+      # TODO: consider using this URL again in future if it ever stabilises and doesn't return random 404s.
+      # raw_url = "https://raw.githubusercontent.com/#{repo}/#{endpoint}"
+      formulae_brew_sh_url = "#{HOMEBREW_API_DEFAULT_DOMAIN}/api/cask-source/#{filepath}"
+
+      output = Utils::Curl.curl_output("--fail", formulae_brew_sh_url)
       raise ArgumentError, "No file found at #{Tty.underline}#{raw_url}#{Tty.reset}" unless output.success?
 
       cache[endpoint] = output.stdout