[5.x] Fix blueprint cache

[aerni]

I noticed that the blueprint fields don't contain new items after using $blueprint->setContents() in a listener. This PR ensures the blueprint cache is reset.

Left without PR. Right with PR. Notice the missing text field on the left.

You can reproduce the issue with this simple listener:

<?php

namespace App\Listeners;

use Statamic\Events\EntryBlueprintFound;

class ExtendBlueprint
{
    public function handle(EntryBlueprintFound $event): void
    {
        $contents = collect($event->blueprint->contents())
            ->dot()
            ->merge([
                'tabs.main.sections.0.fields.4.handle' => 'text',
                'tabs.main.sections.0.fields.4.field.type' => 'text',
            ])
            ->undot()
            ->all();

        dd($event->blueprint->setContents($contents)->fields()->all()->all());
    }
}