Fixed /exec command (don't prefix command with exec binary).

Fixed /exec command (don't prefix command with exec binary).

adding 'exec' prevents to use "script" syntax:
   xds-exec -c xds-config.env -- "cd build && cmake .."
But 'exec' binary is mandatory to allow to pass correctly signals to subprocess such as gdb.
As workaround, exec is set for now on client side (eg. in xds-gdb).