Optimize PR builds: Reduce CI costs by excluding ARM/macOS architectures

[bjoernbethge]

Summary

This PR optimizes CI costs for pull request builds by conditionally excluding expensive architecture targets that are not critical for PR validation.

Changes

• Modified .github/workflows/MainDistributionPipeline.yml to conditionally exclude architectures based on event type
• PR builds: Exclude linux_arm64, osx_amd64, osx_arm64 (in addition to existing WASM exclusions)
• Main branch builds: Continue building all architectures as before

Cost Savings

• ~50% reduction in PR build costs by skipping 3 additional architectures
• Maintains linux_amd64 and windows_amd64 builds for PR validation
• Full architecture matrix still runs on main branch for production releases

Rationale

• Most PR validation can be done on linux_amd64 and windows_amd64
• ARM and macOS-specific issues are rare and can be caught in main branch builds
• Significantly faster PR feedback loop (fewer parallel jobs)

Testing

The conditional logic uses GitHub's standard github.event_name context to differentiate between pull requests and other events (push to main, workflow_dispatch).