fix: close IO sink after download finishes or fails

[bryanoltman]

Description

Close the IOSink we create in ArtifactManager.downloadFile after the write either completes or fails.

Fixes this issue when running shorebird doctor -v:

Network Speed
✓ GCP Upload Speed: 7.08 MB/s (0.9s)
✗ GCP download speed test failed: PathAccessException: Cannot delete file, path = 'C:\Users\bryan\AppData\Local\Temp\71835ee1\speed_test_file' (OS Error: The process cannot access the file because it is being used by another process.
, errno = 32) (1.3s)

✓ Shorebird is up-to-date (1.5s)
✓ Flutter install is correct (2.4s)

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
• 🧪 Tests

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

📢 Thoughts on this report? Let us know!