gh-actions/test-on-iotlab: install riotctrl
gh-actions/test-on-iotlab: install riotctrl

For actually running the test, the docker container is not used. As
such, `riotctrl` needs to be installed for the tests to succeed.

jasonatran/RIOT,authmillenon/RIOT,authmillenon/RIOT,kaspar030/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,kaspar030/RIOT,authmillenon/RIOT,kYc0o/RIOT,miri64/RIOT,kYc0o/RIOT,jasonatran/RIOT,ant9000/RIOT,OTAkeys/RIOT,miri64/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,OlegHahm/RIOT,jasonatran/RIOT,ant9000/RIOT,OTAkeys/RIOT,miri64/RIOT,kYc0o/RIOT,miri64/RIOT,ant9000/RIOT,kYc0o/RIOT,kaspar030/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,authmillenon/RIOT,authmillenon/RIOT,ant9000/RIOT,jasonatran/RIOT,OlegHahm/RIOT,OTAkeys/RIOT,ant9000/RIOT,kaspar030/RIOT,miri64/RIOT,OTAkeys/RIOT