Bug 1291473: mark valgrind builds explicitly as tier-1; r=kmoir

MozReview-Commit-ID: 3brmQIyjXEj

--HG--
extra : rebase_source : 0bdfdb8f2212ec906bd96a70d817e5e9791f90c0

taskcluster/ci/legacy/tasks/builds/opt_linux64_valgrind.yml

@@ -28,4 +28,5 @@ task:
       symbol: V
       machine:
         # see https://github.com/mozilla/treeherder/blob/master/ui/js/values.js
-        platform: linux64
+        platform: linux64
+      tier: 1