Push fails when running from pull requests from a different user

[felds]

Although committing and pushing from the same account is possible with v2, whenever the workflow is running from a pull request made by a different user, the HEAD comes as detached and pushing back is not possible.

Steps to reproduce:

1. Create new a repo with user 🐮 (ex: 🐮/date-test)
2. Add the following workflow to the repo:

   on: [pull_request]
   
   jobs:
     add_date:
       runs-on: ubuntu-latest
       steps:
         - name: Checkout
           uses: actions/checkout@v2
           with:
             fetch-depth: 0
         - name: Add date
           run: date > date.txt
         - name: Push it 
           run: |
             git config --global user.email "[email protected]"
             git config --global user.name "GitHub Action"
             git add date.txt
             git commit -m "Add date"
             git push

3. Fork the repo from user 🐷;
4. Make some changes, commit, and create a new PR.