fix(values): set image pull secrets at correct key

General MR

Summary

This MR updates the image pull secrets for the blackbox exporter to make sure they're provided in the correct key based on the upstream templates: https://github.com/prometheus-community/helm-charts/blob/896a6c3f8ef01d66002f3734d942432723c4104e/charts/prometheus-blackbox-exporter/templates/_helpers.tpl#L138

Relevant logs/screenshots

N/A

Linked Issue

#295 (closed)

Upgrade Notices

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