Add `-Xswiftc -cross-module-optimization` for release?

Test `-Xswiftc -cross-module-optimization` compilation flag for release.

Described in:
- [vapor](https://github.com/vapor/template/blob/main/Package.swift#L25)
- [swift-server](https://github.com/swift-server/guides/blob/main/docs/building.md#building-for-production)
