[infra] trigger kokoro builds w/ the “kokoro:run” label (#370)

Fixes: #369 

---

<details>
  <summary>Contribution guidelines:</summary><br>

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Author: pq

.github/dependabot.yml

@@ -6,6 +6,7 @@ updates:
       interval: "weekly"
     labels:
       - "autosubmit"
+      - "kokoro:run"
     # Updating patch versions for "github-actions" is too chatty.
     # See https://github.com/flutter/flutter/issues/158350.
     ignore:
@@ -18,3 +19,5 @@ updates:
       interval: "weekly"
     labels:
       - "autosubmit"
+      - "kokoro:run"
+