benchmarking: fix script MOOSE_DIR guessing
benchmarking: fix script MOOSE_DIR guessing

Use the benchmark.py's path directly rather than assuming we can get it
from sys.argv - which breaks when doing e.g. "python benchmark.py"
instead of "./benchmark.py".

ref #9834

permcody/moose,laagesen/moose,idaholab/moose,jessecarterMOOSE/moose,milljm/moose,jessecarterMOOSE/moose,lindsayad/moose,bwspenc/moose,dschwen/moose,dschwen/moose,milljm/moose,laagesen/moose,lindsayad/moose,bwspenc/moose,YaqiWang/moose,sapitts/moose,jessecarterMOOSE/moose,lindsayad/moose,harterj/moose,permcody/moose,idaholab/moose,nuclear-wizard/moose,YaqiWang/moose,sapitts/moose,friedmud/moose,idaholab/moose,harterj/moose,harterj/moose,jessecarterMOOSE/moose,dschwen/moose,permcody/moose,andrsd/moose,friedmud/moose,milljm/moose,idaholab/moose,YaqiWang/moose,SudiptaBiswas/moose,andrsd/moose,dschwen/moose,nuclear-wizard/moose,laagesen/moose,permcody/moose,bwspenc/moose,laagesen/moose,andrsd/moose,SudiptaBiswas/moose,andrsd/moose,milljm/moose,dschwen/moose,nuclear-wizard/moose,SudiptaBiswas/moose,lindsayad/moose,friedmud/moose,jessecarterMOOSE/moose,SudiptaBiswas/moose,sapitts/moose,SudiptaBiswas/moose,milljm/moose,idaholab/moose,andrsd/moose,sapitts/moose,friedmud/moose,bwspenc/moose,harterj/moose,laagesen/moose,sapitts/moose,YaqiWang/moose,lindsayad/moose,bwspenc/moose,nuclear-wizard/moose,harterj/moose