Remove "Warm Step/Build Cache" ECR Step

[samchungy]

Through testing with pnpm on a number of my team's repos. I found that installing the deps per step without a cache step was in most cases quicker/almost the same amount of time as pulling from ECR.

When/if the build agency folks add a persistent EFS between build agents, this would completely forego the need for ECR as the cache. If this happens, we can utilise Docker's cache mounting and finally reap the full benefits of the pnpm store.

RUN --mount=type=cache,id=pnpm,target=/pnpm/store pnpm install --prod --frozen-lockfile

[72636c]

I think I could be convinced to do this once we land #1289; I'd have concerns around depending that heavily on the Yarn mirror to rebuild in a DR scenario given past flakiness.