build_artifact: Use r for regular expression. pylint error anomalous-backslash-in-string. BUG=chromium:1003986 TEST=unittest, cros flash, devserver_integration_test Change-Id: I4514c42648977fd4694d20e43ad70f48cf284adb Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/1835574 Tested-by: Achuith Bhandarkar <achuith@chromium.org> Reviewed-by: Amin Hassani <ahassani@chromium.org> Reviewed-by: Mike Frysinger <vapier@chromium.org> Commit-Queue: Achuith Bhandarkar <achuith@chromium.org>