Fix static code error in Searchable\Aggregator.php

[jnoordsij]

• Laravel version: latest (CI)
• Algolia Scout Extended version: latest (CI)
• Algolia Client Version: n/a
• Language Version: any

Description

With the latest larastan version installed, the following error appears when running static analysis:

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  Line   Searchable\Aggregator.php
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  266    Call to an undefined method Illuminate\Database\Eloquent\Model::searchableUsing().
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A possible solution would be working with the laravel/scout repository to add a proper contract to allow for better type-hints; see also laravel/scout#771.

Steps To Reproduce

Run composer update followed by composer test.

See any recently created CI test job, e.g. https://app.circleci.com/pipelines/github/algolia/scout-extended/255/workflows/ec6dc973-ce16-4d43-82e8-a61c2b2b4cd4/jobs/2079/parallel-runs/0/steps/0-109.