[build-tools] Add ios_signing_backend option to the repack step

[gabrieldonadel]

Why

We recently added experimental support for running repack for iOS apps on Linux machines starting from the package version 0.9.0. The idea for using Linux machines is that these are cheaper and more available than macOS ones.

This PR exposes that backend as an opt-in input on the eas/repack step so we can test it end-to-end on EAS. A follow-up PR in universe adds the matching ios_signing_backend param to workflow repack jobs.

How

• Adds optional ios_signing_backend input to eas/repack (fastlane | zsign). Default stays fastlane
• Bumped the @expo/repack-app devDependency to ~0.9.0 for the backend-discriminated IosSigningOptions union. Even though the runtime package is still installed dynamically per job via repack_version.

Test Plan

• Added a new unit test: resolving zsign options writes the .p12 and profiles to the temp dir, returns backend: 'zsign', and never touches IosCredentialsManager.
• After this lands: run a workflow repack job with ios_signing_backend: zsign and runs_on: linux-medium against a real build.

Note: v0.9.0 is published as next on npm, so jobs opting this require setting repack_version: next for now

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 63.75%. Comparing base (ce93a92) to head (150c631).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4239      +/-   ##
==========================================
+ Coverage   63.73%   63.75%   +0.03%     
==========================================
  Files        1030     1030              
  Lines       47448    47491      +43     
  Branches     9977     9991      +14     
==========================================
+ Hits        30237    30274      +37     
- Misses      17110    17116       +6     
  Partials      101      101              

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[github-actions]

✅ Thank you for adding the changelog entry!