fix: format log message correctly for heartbeat

[vigneshshanmugam]

Summary

• Fix the formatting of the logs on HB for project monitors.

Running %s in %s/usr/share/heartbeat/.node/node/bin/npm install --no-audit --no-update-notifier --no-fund --package-lock=false --progress=false/tmp/elastic-synthetics-unzip-4066455738

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

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Pinging @elastic/obs-ds-hosted-services (Team:obs-ds-hosted-services)

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• Duration: 54 min 40 sec

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[emilioalvap]

LGTM, good catch!