license-header.rb: Don't read UTF-8 BOM as text when checking headers

.ci/license-header.rb

@@ -18,7 +18,7 @@ print 'Checking files...'
 issue_files = []
 branch_changed_files.each do |file_name|
    if file_name.end_with?('.cpp', '.h', '.kt', '.kts') and File.file?(file_name)
-      file_content = File.read(file_name)
+      file_content = File.read(file_name, mode: 'r:bom|utf-8')
       if not file_content.start_with?(license_header)
          issue_files.push(file_name)
       end