fix: Use heredoc for multiline JSON output in canary workflow

[tokio-on-jupiter]

Summary

• Fix GitHub Actions output for multiline JSON from lerna changed --json
• Use heredoc delimiter syntax (<<EOF) for multiline output values

Problem

GitHub Actions $GITHUB_OUTPUT doesn't handle multiline values with simple echo. The JSON output from lerna contains newlines which caused: Invalid format ' {'

Test plan

• Merge this PR
• Trigger /canary-release on PR feat(PLATENG-800): Replace @lifeomic/alpha with @jupiterone/platform-sdk-fetch #1188

[Copilot]

Pull request overview

This PR fixes a GitHub Actions workflow issue where multiline JSON output from lerna changed --json was not being properly handled by $GITHUB_OUTPUT. The solution implements heredoc delimiter syntax to correctly capture multiline output values.

Changes:

• Replaced single-line echo assignment with heredoc delimiter syntax (<<EOF) for the packages output variable
• Added proper multiline handling to prevent "Invalid format" errors in GitHub Actions

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