[worker] Use cache API for the legacy cache

[sjchmiela]

The legacy cache does not work in workflow-orchestration.

This rewrites the CacheManager to the new/not-so-new Turtle caches API.

Summary

• replace worker build cache restore/save GCS config URLs with turtle cache download/upload helpers
• derive stable build cache keys without mutating job.cache and preserve clear for force uploads
• keep existing tar archive restore/save semantics and add CacheManager unit coverage

Test plan

• cd packages/worker && yarn test CacheManager.test.ts
• cd packages/worker && yarn typecheck
• cd packages/build-tools && yarn typecheck
• git diff --check

[github-actions]

Subscribed to pull request

File Patterns	Mentions
**/*	@douglowder

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[github-actions]

✅ Thank you for adding the changelog entry!

[codecov]

Codecov Report

❌ Patch coverage is 93.02326% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 58.45%. Comparing base (f8c2b2d) to head (e11ae03).

Files with missing lines	Patch %	Lines
packages/worker/src/CacheManager.ts	92.60%	2 Missing ⚠️
...kages/build-tools/src/steps/functions/saveCache.ts	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3860      +/-   ##
==========================================
+ Coverage   58.42%   58.45%   +0.04%     
==========================================
  Files         917      917              
  Lines       39978    39985       +7     
  Branches     8416     8416              
==========================================
+ Hits        23353    23370      +17     
+ Misses      16530    16520      -10     
  Partials       95       95              

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