sim: cris: always search for local rvdummy tool

sim: cris: always search for local rvdummy tool

If the board info sets the sim to a basename that is found via $PATH
(which is the default dejagnu behavior), the logic here to use its
dirname to find rvdummy fails because it looks for `./rvdummy`.  So
switch it to always use the local build of rvdummy which is the one
we want to be testing against in the first place.

If we get a request for testing against a different setup, we can
figure out & document the needs at that point, and then setup some
config knobs to control it.