ci: fix iOS builds

[kirillzyusko]

📜 Description

Fixing CI failures for iOS e2e jobs (building iOS 26 app).

💡 Motivation and Context

XCode 26.1 can not build app anymore: https://github.com/kirillzyusko/react-native-keyboard-controller/actions/runs/26761419312/job/78889726965

So in this PR I'm switching to the same version as I have on my PC (26.4). We can not switch iOS simulator to iOS/XCode 26.4 because we use macos-15 for runners (due to instability of macos-26) and macos-15 doesn't support xcode 26.4

📢 Changelog

CI

• use Xcode 26.4 for building tests for iOS 26;

🤔 How Has This Been Tested?

Tested manually via this PR.

📸 Screenshots (if appropriate):

📝 Checklist

• CI successfully passed
• I added new mocks and corresponding unit-tests if library API was changed

[github-actions]

📊 Package size report

Current size	Target Size	Difference
317920 bytes	317920 bytes	0 bytes 📉

[github-actions]

1. Invalid Xcode Version Specified
   Why: The workflow specifies xcode: "26.4" which is an invalid version as Xcode versions do not go that high yet.
   Fix: Replace with a valid Xcode version like "15.0" or the latest available.

2. iOS Version Mismatch
   Why: The matrix defines ios: 26 and artifact: "26.4" which are invalid iOS versions, as Apple hasn't released such versions yet.
   Fix: Use a valid iOS version like "16.0" or adjust the artifact version accordingly.