Bug 1533071 - Fix openh264 win64-aarch64 platform so signing can find it. r=dminor a=apavel|sheriffduty

Differential Revision: https://phabricator.services.mozilla.com/D25103

--HG--
extra : rebase_source : cf48f1e8b6d42ce59297c28c13e84e3f7cc263ae

taskcluster/ci/openh264-plugin/kind.yml

@@ -132,7 +132,7 @@ jobs:
             - win64-clang-cl
     win64-aarch64/opt:
         attributes:
-            build_platform: windows2012-aarch64
+            build_platform: win64-aarch64
             build_type: opt
         treeherder:
             platform: win64-aarch64/opt