Support symfony/dotenv 8

[kAlvaro]

Fixes #55

[kAlvaro]

@helhum I don't know what I'm doing. 😅
Not sure how to validate this. I tried running tests locally on my machine but I'm getting permission errors... on Windows!

PS C:\Proyectos\Software\Terceros\dotenv-connector> .\vendor\bin\phpunit tests                                                                            
PHPUnit 9.6.34 by Sebastian Bergmann and contributors.

....FEE                                                             7 / 7 (100%)

Time: 00:00.073, Memory: 6.00 MB

There were 2 errors:

1) Helhum\DotEnvConnector\Tests\Unit\IncludeFileTest::includingFileDoesNotOverrideExistingEnvVars
rmdir(C:\Proyectos\Software\Terceros\dotenv-connector\tests\Unit/Fixtures/foo): Permission denied

C:\Proyectos\Software\Terceros\dotenv-connector\tests\Unit\IncludeFileTest.php:26

2) Helhum\DotEnvConnector\Tests\Unit\IncludeFileTest::includingFileDoesOverrideExistingEnvVars
rmdir(C:\Proyectos\Software\Terceros\dotenv-connector\tests\Unit/Fixtures/foo): Permission denied

C:\Proyectos\Software\Terceros\dotenv-connector\tests\Unit\IncludeFileTest.php:26

--

There was 1 failure:

1) Helhum\DotEnvConnector\Tests\Unit\IncludeFileTest::dumpReturnsFalseIfFileCannotBeWritten
Failed asserting that true is false.

C:\Proyectos\Software\Terceros\dotenv-connector\tests\Unit\IncludeFileTest.php:134

ERRORS!
Tests: 7, Assertions: 18, Errors: 2, Failures: 1.