Avoid creating xrun.log files in core_ibex directory

This is a bit of an overlap with the existing sim.log files that we're
creating (which contain slightly more information), but it turns out
that if you don't say anything then you get an xrun.log in the current
working directory.

dv/uvm/core_ibex/yaml/rtl_simulation.yaml

@@ -180,6 +180,7 @@
            +UVM_VERBOSITY=UVM_LOW
            +bin=<binary>
            +ibex_tracer_file_base=<sim_dir>/trace_core
+           -l <sim_dir>/xrun.log
            -nokey
            <sim_opts>
            <cov_opts>