ci/qemu: allow this workflow to be manually triggered

This patch adds a way to trigger this workflow by navigating to the
repository's Actions tab and selecting this QEMU workflow.

Signed-off-by: Vincent Fu <vincent.fu@samsung.com>

.github/workflows/qemu.yml

@@ -1,6 +1,7 @@
 name: CI
 
 on:
+  workflow_dispatch:
   schedule:
     - cron: "50 3 * * *"  # daily at 4:50 UTC (00:50 EST)