fix(shorebird_cli): don't export FLUTTER_STORAGE_BASE_URL in shorebird.ps1

[bryanoltman]

Description

Invoke flutter --version in a "subshell" to avoid polluting the powershell environment.

Fixes https://github.com/shorebirdtech/shorebird/issues/2108

Type of Change

• [ ] ✨ New feature (non-breaking change which adds functionality)
• [x] 🛠️ 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