system_config: hide HOMEBREW_BUNDLE_* env vars.

These are being introduced in e.g.
https://github.com/Homebrew/homebrew-bundle/pull/486 and we don't care
about them in Homebrew bug reports.

Library/Homebrew/system_config.rb

@@ -169,6 +169,7 @@ class SystemConfig
       unless ENV["HOMEBREW_ENV"]
         ENV.sort.each do |key, value|
           next unless key.start_with?("HOMEBREW_")
+          next if key.start_with?("HOMEBREW_BUNDLE_")
           next if boring_keys.include?(key)
           next if defaults_hash[key.to_sym]