How to skip steps or jobs in GitHub Actions for PRs from forks

​Encrypted secrets in GitHub actions aren't available for builds from forks, so if your build script includes PRs, like Avatar:

on: 
  push:
    branches: [ main, dev, 'feature/*', 'rel/*' ]
  pull_request:
    types: [opened, synchronize, reopened]

You may need to limit steps or entire jobs from running when PRs are coming from forks due to missing secrets (i.e. pushing an output package to a CI nuget feed, say).

The "magic" thing is to use an if condition with the following expression:

  push:
    name: push nuget.ci
    runs-on: ubuntu-latest
    needs: [build, acceptance]
    if: ${{ !github.event.pull_request.head.repo.fork }}
    steps:

Note that in the above case, an entire job is skipped, but you can also apply it to a step instead.