Bug 1530908 - Always set USE_ARTIFACT from taskcluster for artifact builds. r=chmanchester

The artifact builds that are automatically derived using the artifact
template set the USE_ARTIFACT environment variable from taskcluster.
After the previous change, --artifact builds from try syntax do that
too.
That leaves us with only the artifact-build build not doing it, so for
consistency, do it there. That makes it not necessary to set
USE_ARTIFACT from mozconfig.artifact.automation anymore.

Depends on D22056

Differential Revision: https://phabricator.services.mozilla.com/D21313

--HG--
extra : moz-landing-system : lando

build/mozconfig.artifact.automation

@@ -5,4 +5,3 @@ MOZ_AUTOMATION_BUILD_SYMBOLS=0
 MOZ_AUTOMATION_L10N_CHECK=0
 MOZ_AUTOMATION_PACKAGE_GENERATED_SOURCES=0
 MOZ_AUTOMATION_ARTIFACT_BUILDS=1
-USE_ARTIFACT=1

taskcluster/ci/artifact-build/kind.yml

@@ -32,6 +32,7 @@ jobs:
             env:
                 PERFHERDER_EXTRA_OPTIONS: artifact
                 MOZ_ARTIFACT_TASK: {task-reference: '<linux64-opt>'}
+                USE_ARTIFACT: '1'
         run:
             using: mozharness
             actions: [get-secrets, build]