Don't run npm install when in an Actions workflow

[mbg]

Follow-up to #3154 to skip running npm install when running in a GitHub Actions workflow.

Risk assessment

For internal use only. Please select the risk level of this change:

• Low risk: Changes are fully under feature flags, or have been fully tested and validated in pre-production environments and are highly observable, or are documentation or test only.

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Consider adding a changelog entry for this change.
• Confirm the readme and docs have been updated if necessary.

[Copilot]

Pull Request Overview

This change modifies the check-node-modules.sh script to skip running npm install when executing in a GitHub Actions workflow environment, addressing potential issues with dependency installation in CI environments.

• Adds a check for the GITHUB_ACTIONS environment variable to exit early when running in CI
• Improves logging to provide clear feedback about npm install decisions
• Maintains existing logic for local development environments