[shellcheck] Re-enable shellcheckrc

### TL;DR

Un-revert https://github.com/abcxyz/actions/pull/58 (which reverted https://github.com/abcxyz/actions/pull/26 and https://github.com/abcxyz/actions/pull/54) due to an edge case not caught in testing. We rolled back to unbreak the build, but we still want the feature to disable certain rules.

### Expected behavior

_No response_

### Observed behavior

_No response_

### Environment Details

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GitHub Actions
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### Additional information

_No response_

Review the linked pull requests #58, #26, and #54 to understand the shellcheckrc feature and the edge case that caused the revert. Look for shellcheck configuration files and GitHub Actions workflow files in the repository. The goal is to re-enable the feature safely, ensuring the build does not break.

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