Update laravel-logger.php

[FeBe95]

We copied the config to our own /config directory and PHPStan reported the following error:

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  Line   laravel-logger.php                                   
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  30     Expression on left side of ?? is not nullable.       
         🪪 nullCoalesce.expr                                 
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This happens because the (string) cast has a higher operator precedence than the ?? null coalescing operator.

I changed the code to use the built-in fallback of the env() function instead.