Bug 1869860 - Set up "cron" decision task to run tb-rust automation. r=dandarnell

A bit of a hack to generate a Taskcluster hook that will trigger the tb-rust
vendor automation job. The cron task will never actually run with an empty
schedule, but with appropriate grants in ci-configuration it will be triggerable
by a CI job on mozilla-central and by Thunderbird sheriffs & releng.

Differential Revision: https://phabricator.services.mozilla.com/D199632

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

.cron.yml

@@ -96,3 +96,12 @@ jobs:
           - comm-central
       when: [{hour: 12, minute: 0}]
       # when: []
+
+    - name: tb-rust-vendor-check
+      job:
+          type: decision-task
+          treeherder-symbol: tb-rust-vendor-check
+          target-tasks-method: tb_rust_vendor_check
+      run-on-projects:
+          - comm-central
+      when: []  # never (hook only)

taskcluster/comm_taskgraph/target_tasks.py

@@ -73,3 +73,13 @@ def target_tasks_ash(full_task_graph, parameters, graph_config):
         )
 
     return [l for l, t in full_task_graph.tasks.items() if _filter(t, parameters)]
+
+
+@_target_task("tb_rust_vendor_check")
+def target_tasks_tb_rust_vendor_check(full_task_graph, parameters, graph_config):
+    """Select the set of tasks required to run tb-rust check-upstream & vendor."""
+
+    def _filter(task):
+        return task.kind in ["repo-update"]
+
+    return [l for l, t in full_task_graph.tasks.items() if _filter(t)]