jenkins: rm everything except git dir (#21658)

selfdrive/test/setup_device_ci.sh

@@ -26,6 +26,9 @@ fi
 
 # set up environment
 cd $SOURCE_DIR
+git reset --hard
+git fetch
+find . -maxdepth 1 -not -path './.git' -not -name '.' -not -name '..' -exec rm -rf '{}' \;
 git fetch --verbose origin $GIT_COMMIT
 git reset --hard $GIT_COMMIT
 git checkout $GIT_COMMIT