CI: Use number of re-run attempts for Percy nonce

[swissspidy]

Context

#11983 (comment)

Summary

github.run_attempt is a unique number for each attempt of a particular workflow run in a repository, starting at 1 and incrementing with each re-run.

Using this number for workflow re-runs avoids Percy warning “the build is already finalized”, because otherwise the nonce stays the same between runs.

Relevant Technical Choices

To-do

User-facing changes

Testing Instructions

• This is a non-user-facing change and requires no QA

This PR can be tested by following these steps:

Reviews

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Checklist

• This PR addresses an existing issue and I have linked this PR to it in ZenHub
• I have tested this code to the best of my abilities
• I have verified accessibility to the best of my abilities (docs)
• I have verified i18n and l10n (translation, right-to-left layout) to the best of my abilities
• This code is covered by automated tests (unit, integration, and/or e2e) to verify it works as intended (docs)
• I have added documentation where necessary
• I have added a matching Type: XYZ label to the PR

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Fixes #