Override defaultTestDevice for test-builds-xcode-26-release (#5453)

## Summary
test-builds-xcode-26-release is failing due to not being able to access
the 16.4 simulator. I believe i experienced the same issue with the
other job, and forgot that we need to override the env variable.

## Motivation
CI Blocked.

## Testing
None.

## Changelog
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Author: wooj-stripe

bitrise.yml

@@ -401,6 +401,8 @@ workflows:
         inputs:
         - content: xcodebuild build -workspace "Stripe.xcworkspace" -scheme "AllStripeFrameworks" -configuration "Release" -sdk "iphonesimulator" | xcpretty
         title: Build release builds
+    envs:
+    - DEFAULT_TEST_DEVICE: platform=iOS Simulator,name=iPhone 16,OS=26.0
     before_run:
     - prep_all
     meta: