luci: use Python3 on LUCI

Bug: crbug/1313718
Change-Id: I0c5e8904bf1c9a01b86dd8eada4c48b6b005dcf3
diff --git a/infra/luci/main.star b/infra/luci/main.star
index 316a505..e7c01e4 100755
--- a/infra/luci/main.star
+++ b/infra/luci/main.star
@@ -82,6 +82,7 @@
             name = "perfetto",
             cipd_package = "infra/recipe_bundles/android.googlesource.com/platform/external/perfetto",
             cipd_version = "refs/heads/master",
+            use_python3 = True,
         ),
         dimensions = {
             "pool": "luci.perfetto.official",