github workflows: Stick with `which` instead of `command -v`
github workflows: Stick with `which` instead of `command -v`

... because even though it looks like a subshell, it apparently is not
evaluated through a shell but passed right into exec.

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