[11.x] Filter environment variables in ServeCommand

[mpociot]

This PR fixes an old outstanding issue that previously required a php.ini adjustment. (https://github.com/laravel/framework/issues/34229)

At the moment, the ServeCommand does not really filter environment variables that are being passed to the PHP process, but instead it sets them to false. This can lead to issues if the variable is being used by the system and is then set to false.

Instead of passing the variables as false, this PR now filters the env variables and only passes those to the process that are passthrough variables - or all in case --no-reload is specified.

[crynobone]

What's would a good example for environment variables with values as false?

Alternatively, would it make sense if we encode $value using something like:

https://github.com/orchestral/testbench-core/blob/37ed7cca1603c57e729388e7df6c13f87ba37bbb/src/Foundation/Env.php#L64-L85

[taylorotwell]

Drafting pending feedback from @mpociot