Boost: Prevent the same LCP job from being added multiple times

[dilirity]

Fixes HOG-396

We do this for Cloud CSS as well, but I didn't want to touch it as we have job deferring and the payload is merged.

Proposed changes:

• Simplify the job ID to prevent the same job from being enqueued multiple times.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

HOG-396

Does this pull request change what data or activity we track or use?

no

Testing instructions:

• This is easiest tested with local boost cloud;
• Setup Boost and enable LCP;
• Keep an eye on the logs - there should be entries about the job being enqueued and processed;
• Stop the worker for the LCP service (boost-hydra-lcp). This will prevent the jobs from being processed and you'll be able to observe the queue easily;
• Toggle the LCP analysis multiple times and pay attention to the request ID;
• If you're using trunk/production Boost, you should see a different request ID every time you toggle the feature;
• If you're using this branch, you should see the same request ID when you toggle the feature. That's assuming you didn't update the cornerstone pages list.

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Code Coverage Summary

This PR did not change code coverage!

That could be good or bad, depending on the situation. Everything covered before, and still is? Great! Nothing was covered before? Not so great. 🤷

Full summary · PHP report · JS report

_{Coverage check overridden by I don't care about code coverage for this PR Use this label to ignore the check for insufficient code coveage. .}

[dilirity]

Merging this as it's a trivial change.