Symfony DotEnv version(s) affected: v4.3.6 up until at least v4.3.8

Description
Using environment variables in .env files fails.

How to reproduce

• Create .env file with the following content:

MY_HOSTNAME=${HOSTNAME}

where HOSTNAME is an existing environment variable.

• Expected result: MY_HOSTNAME will have the value of the HOSTNAME environment variable.
• Actual result : MY_HOSTNAME has no value.

Additional context
I ran into this problem in the context of TYPO3 9LTS with dependency dotenv-connector:v2.1.2 which has a dependency on symfony/dotenv:^4.0