Add a workflow to keep other scheduled workflows alive

GitHub disables scheduled workflows after 60 days of inactivity on a
repository. Work around this by adding a scheduled workflow that creates
a small commit once per month.

Ported-from: https://github.com/git-for-windows/track-website-changes/blob/main/.github/workflows/keepalive.yml
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

.github/workflows/keepalive.yml

@@ -0,0 +1,36 @@
+name: Keep scheduled workflows alive
+
+on:
+  schedule:
+    - cron: "23 19 17 * *" # 7:23pm on every 17th of the month
+  workflow_dispatch:
+
+jobs:
+  job:
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+    permissions:
+      contents: write
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          persist-credentials: 'true'
+      - name: Create and push a commit
+        env:
+          GIT_CONFIG_PARAMETERS: "'user.name=CI' 'user.email=ci@github'"
+        run: |
+          mkdir -p .github/cached
+          file='.github/cached/keepalive.txt'
+          date >$file
+          git add "$file"
+          git commit -m "workflow keepalive" &&
+          git push origin HEAD </dev/null || {
+            for i in 1 2 3 4 5
+            do
+              # In case of concurrent pushes, let's pull and push
+              git pull origin $GITHUB_REF </dev/null || exit 1
+              git push origin HEAD </dev/null && exit 0
+            done
+            exit 1
+          }