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>
1 file changed