Bug 1833583 - Use taskcluster-built python for valgrind task r=glandium

Differential Revision: https://phabricator.services.mozilla.com/D178288

taskcluster/ci/valgrind/kind.yml

@@ -16,6 +16,7 @@ transforms:
 job-defaults:
     attributes:
         retrigger: true
+    use-system-python: false
 
 jobs:
     linux64-valgrind-qr/opt-swr: