download_strategy: don't check url if already downloaded

Closes Homebrew/homebrew#40859.

Signed-off-by: Xu Cheng <xucheng@me.com>

Library/Homebrew/download_strategy.rb

@@ -270,6 +270,7 @@ class CurlDownloadStrategy < AbstractFileDownloadStrategy
   def fetch
     ohai "Downloading #{@url}"
 
+    unless cached_location.exist?
       urls = actual_urls
       unless urls.empty?
         ohai "Downloading from: #{urls.last}"
@@ -279,7 +280,6 @@ class CurlDownloadStrategy < AbstractFileDownloadStrategy
         end
       end
 
-    unless cached_location.exist?
       had_incomplete_download = temporary_path.exist?
       begin
         _fetch