keepalive: avoid diverging commits in forks

In forks of `gitgitgadget-workflows`, we want to prefer merging the
upstream branch over generating keepalive commits that diverge from
upstream.

To that end, run the workflow every week, and ensure that there is
a commit at least every 4 weeks in forks, but every 3 weeks in the
upstream repository. That way, the forks should always pick up the
upstream's keepalive commit.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

.github/workflows/keepalive.yml

@@ -2,7 +2,7 @@ name: Keep scheduled workflows alive
 
 on:
   schedule:
-    - cron: "23 19 3,17 * *" # 7:23pm on every 3rd and 17th of the month
+    - cron: "23 19 * * 0" # 7:23pm on every Sunday
   workflow_dispatch:
 
 env:
@@ -34,7 +34,7 @@ jobs:
         if: steps.merge.outputs.result != 'merged'
         run: |
           if test workflow_dispatch != '${{ github.event_name }}' &&
-            test 0 -lt $(git rev-list --count --since=3.weeks.ago HEAD)
+            test 0 -lt $(git rev-list --count --since=${{ github.event.repository.fork == true && '4' || '3' }}.weeks.ago HEAD)
           then
             echo "::notice::No need to keep alive, there were commits in the last three weeks"
             echo "result=skip-push" >>$GITHUB_OUTPUT