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>
diff --git a/build_artifact.py b/build_artifact.py
index fb62773..473bfb0 100755
--- a/build_artifact.py
+++ b/build_artifact.py
@@ -623,7 +623,7 @@
 
 
 _AddCrOSArtifact(artifact_info.FULL_PAYLOAD, AUTestPayload,
-                 '.*full.*bin(\.json)?\\Z', is_regex_name=True)
+                 r'.*full.*bin(\.json)?\Z', is_regex_name=True)
 
 
 class DeltaPayloadNtoN(DeltaPayloadBase):