[6.x] Speed up runningInConsole method

[GrahamCampbell]

This code is actually quite hot, since it's called by most (package) service providers.

The new code is faster because:

1. Env::get isn't all that cheap, in the case where the env variable was found in the last place we look, and so we should only call it once, and assign the result.
2. The php optimizer can now ahead of time replace return php_sapi_name() === 'cli' || php_sapi_name() === 'phpdbg'; with either return true; or return false;, since it is able to replace the constant (that it knows is global due to the slash) with the string value, and then it can evaluate the equality and or, all ahead of time.

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I detected this optimization when profiling a Laravel app with xdebug tracing btw. ;)

[GrahamCampbell]

We can do one better, actually, by caching the computation result, so we don't have to check the environment variable each time the method is called.