Add Timezone support

[jhoff]

Had a teammate see the timezone() method and expected it to work like the Kernel scheduler, but instead our log files just started filling up with Creation of dynamic property CheckClass::$timezone is deprecated warning on every single request and the checks still ran in UTC, despite the code suggesting otherwise.

Spatie\Health\Checks\Check uses Illuminate\Console\Scheduling\ManagesFrequencies which offers the ability to set $timezone. Spatie\Health\Checks\Check::shouldRun() could easily support timezones in the same way that Illuminate\Console\Scheduling\Event::expressionPasses() does now.

[freekmurze]

I'd accept a PR (with tests) for this 👍