formula.rb: Don't attempt to mirror unstable URLs

If a build is being influenced by an option such as `--HEAD`. Don't attempt
mirrors if a download fails.

Fixes Homebrew/homebrew#7971.

Library/Homebrew/formula.rb

@@ -557,7 +557,9 @@ private
   # For brew-fetch and others.
   def fetch
     downloader = @downloader
-    mirror_list = mirrors
+    # Don't attempt mirrors if this install is not pointed at a "stable" URL.
+    # This can happen when options like `--HEAD` are invoked.
+    mirror_list =  @spec_to_use == @stable ? mirrors : []
 
     # Ensure the cache exists
     HOMEBREW_CACHE.mkpath