[build-tools] Re-enable 3rd party precompiled modules

[sjchmiela]

Summary

• Re-enable EXPO_PRECOMPILED_MODULES_BASE_URL for iOS pod installs so third-party precompiled modules are used alongside Expo modules.
• Keep the env scoped to pod install only, and proxy the base URL through EAS_BUILD_COCOAPODS_CACHE_URL when CocoaPods cache is enabled.
• Update pod install unit coverage for the default storage URL and proxied URL behavior.

Testing

• Added focused unit tests for direct and proxied precompiled modules base URL resolution.
• Ran yarn exec oxfmt on the touched files.
• Ran focused @expo/build-tools pod tests and package typecheck; both passed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 56.52%. Comparing base (64874f7) to head (8ae185a).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3681      +/-   ##
==========================================
+ Coverage   56.51%   56.52%   +0.01%     
==========================================
  Files         887      887              
  Lines       38416    38422       +6     
  Branches     8007     8008       +1     
==========================================
+ Hits        21707    21713       +6     
  Misses      16611    16611              
  Partials       98       98              

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[github-actions]

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[chrfalch]

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