[Dotenv] Variable expansion is not taking into account variables being overwritten in subsequent .env files

.env
```
REDIS_HOST=localhost
LOCK_DSN=redis://${REDIS_HOST}
```
.env.local
```
REDIS_HOST=aaa
```

### What happens now

Dotenv in standard symfony configuration will evaluate `LOCK_DSN` as `redis://localhost`

### What is expected to happen

Dotenv should merge all files first and do variable expansion after that. Expected result would be `redis://aaa`
