Bug 1543956 - Use a fixed mozconfig for linux64-aarch64 builds. r=mshal

Differential Revision: https://phabricator.services.mozilla.com/D27631

--HG--
rename : browser/config/mozconfigs/linux64-aarch64/nightly => browser/config/mozconfigs/linux64-aarch64/opt
extra : moz-landing-system : lando

taskcluster/ci/build/linux.yml

@@ -1121,6 +1121,7 @@ linux64-aarch64/opt:
             mozconfig_platform: linux64-aarch64
         script: "mozharness/scripts/fx_desktop_build.py"
         secrets: true
+        mozconfig-variant: opt
         tooltool-downloads: public
         need-xvfb: true
     toolchains: