ci: add scheduled run of GHA CI

This is to ensure that our CI is not rotting away even if there are no
new PRs or merges. This is especially useful for release branches
which tend to cease working over time due to some external reasons.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

Author: kolyshkin

.github/workflows/scheduled.yml

@@ -0,0 +1,29 @@
+name: scheduled
+on:
+  schedule:
+    # Runs at 00:00 UTC every Sunday, Tuesday, Thursday.
+    - cron: '0 0 * * 0,2,4'
+  workflow_dispatch:
+permissions:
+  contents: read
+  actions: write
+
+jobs:
+  trigger-workflow:
+    strategy:
+      matrix:
+        branch: ["main", "release-1.3"]
+        wf_id: ["validate.yml", "test.yml"]
+    runs-on: ubuntu-latest
+    steps:
+      - name: Trigger ${{ matrix.wf_id }} workflow on ${{ matrix.branch}} branch
+        uses: actions/github-script@v7
+        with:
+          github-token: ${{ secrets.GITHUB_TOKEN }}
+          script: |
+            await github.rest.actions.createWorkflowDispatch({
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              workflow_id: '${{ matrix.wf_id }}',
+              ref: '${{ matrix.branch }}'
+            });