GHSL-2020-335: Unauthorized repository modification or secrets exfiltration in a GitHub workflow of libpasta

on:
...
  pull_request_target:
...
      - uses: actions/checkout@v2
        with:
          # This prevents the Action from persisting the credentials it uses to
          # perform the fetch/checkout to the Runner's local Git config. On
          # `pull_request_target` events, the GITHUB_TOKEN provided to the
          # Runner has Write permissions to the base repository. We do **not**
          # want to allow untrusted code from forks to execute arbitrary Git
          # commands with those elevated permissions.
          #
          # More info:
          # https://github.blog/2020-08-03-github-actions-improvements-for-fork-and-pull-request-workflows/#improvements-for-public-repository-forks
          persist-credentials: false
          # Explicitly setting the `repository` and `ref` inputs ensures that
          # `pull_request_target` events trigger CI runs against the code from
          # the HEAD branch. By default, this Action checks out code from the
          # BASE branch. On `push` events, the `github.event.pull_request` path
          # will yield a null value, and passing nulls to these inputs causes
          # them to fall back to the defaults
          repository: ${{ github.event.pull_request.head.repo.full_name }}
          ref: ${{ github.event.pull_request.head.sha }}
...
      - name: Run test
        run: cargo test --all-targets --features long_tests
...