[2.0.x] Fix crash upon repeated calls to process_subcommands_now_P (#10509)

Marlin/src/gcode/gcode.cpp

    */
   void GcodeSuite::process_subcommands_now_P(const char *pgcode) {
     // Save the parser state
-    char saved_cmd[strlen(parser.command_ptr) + 1];
-    strcpy(saved_cmd, parser.command_ptr);
+    const char * const saved_cmd = parser.command_ptr;
 
     // Process individual commands in string
     while (pgm_read_byte_near(pgcode)) {