Fix environment variable clearing in log tests
Fix environment variable clearing in log tests

This causes all sorts of mayhem in later tests that rely on integration
test environment variables, for example. This was introduced in
fedb9ecb762cb80edb7b71bce5db5c87ee6f1b5b, along with the feature it
tests.

tenstartups/wal-e,wal-e/wal-e,ajmarks/wal-e