Investigate why we are being rate-limited in GHA

[nimalank7]

User Need

As a Platform Engineer
I want our image layers to be cached
so that our builds won't be rate limited and are quicker

Context

We believe we're constantly being ratelimited, and we shouldn't be. This was initially included in #3477 but now spun off into it's own issue

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What’s Needed

• Investigate the root cause as to why GHA is rate-limiting us
• Check that our layer caching is actually working
• If there is an obvious cause (and solution) then fix it (if simple), otherwise...
• ...create a follow-up ticket.

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Acceptance Criteria

• Builds aren't rate limited due to layer caching

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User Comms Plan (if applicable)

Briefly describe how you’ll let users know about the change.

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[dj-maisy]

[Refinement] We've decided that this issue has been addressed through other means (e.g. pulling base images from Docker Hub). We assume our alerting is working and hasn't fired since December.