cerbero: Fix environment leakage between recipes

This bug was added in 461803fe

For env vars edited by the recipe, we shouldn't set the old_env to an
incorrect value which also contains any changes made by the recipe.
@modify_environment will take care to restore the original env when
we return from the build step for those vars.
7 jobs for fix-env-leakage in 56 minutes and 1 second (queued for 2 seconds)