push-apk: set google play track to alpha

This lets us temporarily push nightly builds to the alpha track instead
of production so we can gradually roll out the Logins update.

taskcluster/ci/push-apk/kind.yml

@@ -37,6 +37,7 @@ job-template:
                 beta: fennec-beta
                 release: fennec-production
                 nightly: production
+        google-play-track: alpha
         dep:
             by-level:
                 '3': false

taskcluster/fenix_taskgraph/worker_types.py

@@ -74,6 +74,7 @@ def build_scriptworker_signing_payload(config, task, task_def):
         Required("commit"): bool,
         Required("product"): text_type,
         Required("dep"): bool,
+        Optional("google-play-track"): text_type,
     },
 )
 def build_push_apk_payload(config, task, task_def):
@@ -87,6 +88,8 @@ def build_push_apk_payload(config, task, task_def):
         "commit": worker["commit"],
         "upstreamArtifacts": worker["upstream-artifacts"],
     }
+    if worker.get("google-play-track"):
+        task_def["payload"]["google_play_track"] = worker["google-play-track"]
 
     scope_prefix = config.graph_config["scriptworker"]["scope-prefix"]
     task_def["scopes"].append(