Bug 1638607 - Make diffoscope tasks tier 3 in the short term r=glandium

I expect that we'll be able to undo this (at least for Linux64) after clang-12.

Differential Revision: https://phabricator.services.mozilla.com/D103487

taskcluster/ci/diffoscope/kind.yml

@@ -66,6 +66,7 @@ jobs:
 
     reproducible-linux32:
         symbol: Linux32
+        tier: 3
         new: build-linux-reproduced/opt
         original: build-linux-shippable/opt
         fail-on-diff: true
@@ -81,6 +82,7 @@ jobs:
 
     reproducible-linux64:
         symbol: Linux64
+        tier: 3
         new: build-linux64-reproduced/opt
         original: build-linux64-shippable/opt
         fail-on-diff: true