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 <[email protected]>
Reviewed-by: Amin Hassani <[email protected]>
Reviewed-by: Mike Frysinger <[email protected]>
Commit-Queue: Achuith Bhandarkar <[email protected]>
1 file changed