Restore build on helix for mono iOS + TVOS tests

[ericstj]

See #120589 (comment)

When updating the build images to macOS-15 it caused tests to fail to link on these platforms.

We worked around this so that we could get the build fixed before the deprecation deadline, but we should try to restore the build on helix behavior once helix machines have a newer xcode.

#120589

[dotnet-policy-service]

Tagging subscribers to 'os-ios': @vitek-karas, @kotlarmilos, @steveisok, @akoeplinger
See info in area-owners.md if you want to be subscribed.

[dotnet-policy-service]

Tagging subscribers to 'os-tvos': @vitek-karas, @kotlarmilos, @steveisok, @akoeplinger
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[dotnet-policy-service]

Tagging subscribers to this area: @akoeplinger, @matouskozak, @simonrozsival
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