ci: cache test images for reuse across runs

[G-Rath]

While it only tends to take 2-3 minutes, they completely block our three test jobs and in theory it should be very straightforward to cache our test images as (again, in theory) they should be very stable.

Right now I'm just redownloading the tars and skipping the build entirely, though we could also possibly try re-importing them so that we always do a build but they're available as cached layers, which might be a better middleground

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 65.04%. Comparing base (ac13b3b) to head (449faea). Report is 12 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1757      +/-   ##
==========================================
+ Coverage   64.49%   65.04%   +0.54%     
==========================================
  Files         157      163       +6     
  Lines       15839    15950     +111     
==========================================
+ Hits        10216    10374     +158     
+ Misses       4950     4897      -53     
- Partials      673      679       +6     

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

:rocket: New features to boost your workflow:

• :snowflake: Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• :package: JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[G-Rath]

@another-rex what do you think about this? It's actually technically not working on this pull request as even though its reporting it's pushed up the cache, it never finds it when doing the restore, but I think that's because this is coming from a fork - I'm curious to see what happens if we just land it