fix(shorebird_cli): fix failing test

[bryanoltman]

Description

Creates an xcframework as a directory instead of a file.

Fixes the test failure seen in https://github.com/shorebirdtech/shorebird/actions/runs/6788000286/job/18452058897.

Type of Change

• ✨ New feature (non-breaking change which adds functionality)
• 🛠️ Bug fix (non-breaking change which fixes an issue)
• ❌ Breaking change (fix or feature that would cause existing functionality to change)
• 🧹 Code refactor
• ✅ Build configuration change
• 📝 Documentation
• 🗑️ Chore

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (1a1ac60) 100.00% compared to head (0228136) 100.00%.
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #1461   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          151       151           
  Lines         4579      4580    +1     
=========================================
+ Hits          4579      4580    +1     

Flag	Coverage Δ
shorebird_cli	100.00% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
...s/shorebird_cli/lib/src/shorebird_build_mixin.dart	100.00% <100.00%> (ø)

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