ci: add a skeleton downstream actions workflow

so that I can use this workflow in PRs

.github/workflows/downstream.yml

@@ -0,0 +1,24 @@
+name: downstream
+concurrency:
+  group: "${{github.workflow}}-${{github.ref}}"
+  cancel-in-progress: true
+on:
+  workflow_dispatch:
+  schedule:
+    - cron: "0 7 * * 1,3,5" # At 07:00 on Monday, Wednesday, and Friday # https://crontab.guru/#0_7_*_*_1,3,5
+  push:
+    branches:
+      - main
+      - "v*.*.x"
+    tags:
+      - v*.*.*
+  pull_request:
+    types: [opened, synchronize]
+    branches:
+      - '*'
+
+jobs:
+  skeleton:
+    runs-on: ubuntu-latest
+    steps:
+      - run: echo hello world