Investigate buildpackStore.Get 503 failures

[sophiewigmore]

Context

With the addition of tests for multiple builders, we have recently seen an uptick in 503 Service Unavailable errors in our Github Actions related to buildpackStore.Get. These failures cause release drafting to fail, as well as PRs to get blocked on misleading flaky failures.

Issue

We should investigate the buildpackStore.Get code and figure out if there is a way to add retries to getting buildpacks to avoid failing out. The solution may instead be to change how we run the tests on a higher-level so that buildpacks only are pulled once for the tests across multiple builders. Regardless of the best solution, this should be looked into in order to reduce flakiness in our usually stable integration testing system.