github: trigger main test on push to master

The trigger action sends repository_dispatch events to all
main test repositories of the manifests this repo is part of.

Signed-off-by: Gerwin Klein <gerwin.klein@proofcraft.systems>

.github/workflows/trigger.yml

@@ -0,0 +1,19 @@
+# Copyright 2021, Proofcraft Pty Ltd
+#
+# SPDX-License-Identifier: BSD-2-Clause
+
+# Trigger repository dispatch on main test repos
+name: Trigger
+
+on:
+  push:
+    branches: [master]
+
+jobs:
+  trigger:
+    name: Repository Dispatch
+    runs-on: ubuntu-latest
+    steps:
+    - uses: seL4/ci-actions/trigger@master
+      with:
+        token: ${{ secrets.PRIV_REPO_TOKEN }}