[Lock] The choice of a store name from env vars doesn't work

**Symfony version(s) affected**: 4.1.0

**Description**  

> Symfony\Component\Cache\Exception\InvalidArgumentException: Unsupported DSN: semaphore.

semaphore/flock - error
redis/memcached dsn - ok

**How to reproduce**  
```
parameters:
    env(LOCK_STORE_DSN): 'semaphore'

framework:
    lock: '%env(LOCK_STORE_DSN)%'
```
