chore: Use Cirrus Runners for running unit tests on iOS 18

[itaybre]

Similar to #6820, this enables Cirrus Runners on iOS18 unit tests which are timing out.

Example: https://github.com/getsentry/sentry-cocoa/actions/runs/19471609958/attempts/1?pr=6818

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.827%. Comparing base (e58d7bf) to head (dae316b).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@              Coverage Diff              @@
##              main     #6821       +/-   ##
=============================================
- Coverage   84.831%   84.827%   -0.004%     
=============================================
  Files          453       453               
  Lines        27609     27609               
  Branches     12101     12103        +2     
=============================================
- Hits         23421     23420        -1     
+ Misses        4140      3902      -238     
- Partials        48       287      +239     

see 41 files with indirect coverage changes

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