rumur-run: preference colocated rumur binary over the one in $PATH

As pointed out on Github #193, it is confusing to preference the rumur binary in
your $PATH when you have a system installation but have also built Rumur from
source and installed it elsewhere. In this scenario, rumur-run from the
built-from-source version would still invoke the system rumur binary.

Github: Closes #195 "rumur-run: prefer colocated rumur"