Spike: add OPM_THREAD_LIMIT to iiif_print instead of each individual project

[ShanaLMoore]

Summary

To speed up the process of OCR, we've been setting an env var for each individual project.

Kirk noticed that ESSI has it defined when they call tesseract and thought it'd be nice if we did the same in iiif_print, to remove the step of adding it to each project's yml/rancher.

This ticket is to evaluate if using it inline works, and determine if we should proceed with including it in the iiif_print gem.

Acceptance Criteria

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Testing Instructions

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