GitHub Actions has a useful feature to trigger workflows on a cron schedule. For example:

name: Test

on: 
  push:
  pull_request:
  schedule:
    - cron: "0 6 * * *" # daily at 6am

jobs:
  test:
    runs-on: ubuntu-latest

    steps:
      - uses: actions/checkout@v3
... 

But if a contributor has enabled GitHub Actions on their fork (which I recommend: test your contributions before opening a PR), it also runs the cron on their fork. This not only uses up extra CI resources on the fork:

It also sends a regular email to the contributor when the workflow fails:

Instead, we only need to run the cron for the upstream. For example, for a https://github.com/octocat/hello-world repo, add:

 jobs:
   test:
+    if: ${{ github.repository_owner == 'octocat' || github.event_name != 'schedule' }}
     runs-on: ubuntu-latest

This only runs the schedule trigger for the octocat upstream, and all other triggers run for both upstream and forks.

Also

For simpler workflows that only trigger on a cron, such as stalebot or CodeQL runs, we can add a simpler condition.

For example:

name: Close stale issues

on:
  schedule:
  - cron: "10 0 * * *" # daily at 12:10am

jobs:
  stale:
    runs-on: ubuntu-latest

    steps:
    - name: "Check issues"
      uses: actions/stale@v8
...

We can completely disable it for forks:

 jobs:

   stale:

+    if: github.repository_owner == 'octocat'

     runs-on: ubuntu-latest

PS

Use single quotes in these lines, double quotes are invalid here (Unexpected symbol).

Thanks

To Alex Waygood for the tip.

To the British Library and Flickr Commons for the illustration of a chronograph.