audit: do not warn about reachability of brew mirrored URL

A `brew mirror`ed URL is usually not yet reachable at the time of pull request.

Library/Homebrew/dev-cmd/audit.rb

@@ -1566,6 +1566,9 @@ class ResourceAuditor
 
       strategy = DownloadStrategyDetector.detect(url, using)
       if strategy <= CurlDownloadStrategy && !url.start_with?("file")
+        # A `brew mirror`'ed URL is usually not yet reachable at the time of
+        # pull request.
+        next if url =~ %r{^https://dl.bintray.com/homebrew/mirror/}
         if http_content_problem = FormulaAuditor.check_http_content(url)
           problem http_content_problem
         end