ci: trigger ci-pr-riscv64-linux workflow on unlabeled events

This way, if the ci-riscv64-linux label was added to a PR previously, removing
it will cause the concurrency group of the workflow to cancel the runs triggered
by the label being added.

Author: alexrp

.github/workflows/ci-pr-riscv64-linux.yaml

@@ -6,6 +6,7 @@ on:
       - opened
       - reopened
       - synchronize
+      - unlabeled
 concurrency:
   # Cancels pending runs when a PR gets updated.
   group: riscv64-linux-${{ github.head_ref || github.run_id }}-${{ github.actor }}