[ci skip] Run the toolset with --clean before each continuous run (#3403)

toolset/continuous/tasks/run-benchmarks.sh

@@ -4,5 +4,7 @@
 
 echo Change to benchmark root
 cd $TFB_REPOPARENT/$TFB_REPONAME
+echo Cleaning
+PYTHONPATH=$TFB_REPOPARENT/$TFB_REPONAME python toolset/run-tests.py --clean
 echo Running tests
 PYTHONPATH=$TFB_REPOPARENT/$TFB_REPONAME python toolset/run-tests.py