[fix] make.run - use SEARX_DEBUG=1 instead of modifing settings.yml

Reloading webapp.py when a file is changed exit with error::

    $ make run
    ....
    INFO:werkzeug: * Detected change in 'searx/searx/engines/deviantart.py', reloading
    INFO:werkzeug: * Restarting with stat
    ERROR:searx:server.secret_key is not changed. Please use something else instead of ultrasecretkey.
    make: *** [Makefile:69: run] Error 1

Patch suggested by @dalf [1]

[1] https://github.com/searx/searx/issues/2278#issuecomment-719732579



Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>