do not include $FILENAME in globals

`$FILENAME` is unsafe to include in the globals because it modifies ARGV
and raises an error if it's a file that does not exist. This causes two
issues if rdbg used to debug a command with arguments.
 * when using rdbg the server will crash when running `info globals`, as
   it does not catch the error
 * when using a DAP client that requests globals before the debugged
   program was able to handle its arguments then the ARGV changes and
   the program will not do what's expected

lib/debug/thread_client.rb

@@ -44,7 +44,7 @@ def skip_location?(loc)
   end
 
   module GlobalVariablesHelper
-    SKIP_GLOBAL_LIST = %i[$= $KCODE $-K $SAFE].freeze
+    SKIP_GLOBAL_LIST = %i[$= $KCODE $-K $SAFE $FILENAME].freeze
     def safe_global_variables
       global_variables.reject{|name| SKIP_GLOBAL_LIST.include? name }
     end