Pre-release selftests: set git repo directory as safe

This workflow is currently failing, giving the following error:

   fatal: unsafe repository ('/__w/avocado/avocado' is owned by someone else)
   To add an exception for this directory, call:

          git config --global --add safe.directory /__w/avocado/avocado
   Error: Process completed with exit code 128.

Let's work around this limitation and more recent git behavior and set
the safe mode on the repo directory manually.

Reference: actions/checkout#762
Signed-off-by: Cleber Rosa <crosa@redhat.com>

.github/workflows/prerelease.yml

@@ -76,6 +76,7 @@ jobs:
         run: |
           VERSION=$(python3 setup.py --version 2>/dev/null)
           RPM_RELEASE_NUMBER=$(grep -E '^Release:\s([0-9]+)' python-avocado.spec | sed -E 's/Release:\s([0-9]+).*/\1/')
+          git config --global --add safe.directory `pwd`
           COMMIT_DATE=$(git log -n 1 --pretty='format:%cd' --date='format:%Y%m%d')
           SHORT_COMMIT=$(git rev-parse --short=9 master)
           dnf -y install python3-avocado-${VERSION}-${RPM_RELEASE_NUMBER}.${COMMIT_DATE}git${SHORT_COMMIT}.fc${{env.FEDORA_VERSION}}