CI: Set GIT_REF as an environment variable for safety mutation tests

[devtekve]

This change moves the GIT_REF definition into the environment section, otherwise it's not really used as expected by the script

Note to maintainers

I can also set it like this

GIT_REF: ${{ github.event_name == 'push' && github.ref == format('refs/heads/{0}', github.event.repository.default_branch) && github.event.before || format('origin/{0}', github.event.repository.default_branch) }}

I think this is better for forks in the long run as some (like us at SP for the time being) use master-new for our rewrite with this it becomes more universal