Can't build version 1.2.1 (Draft)

I'm currently having trouble adding/building version 1.2.1 to my project. I tried using 1.2.0 and everything built fine.

**Swift Algorithms version:** `1.2.1`
**Swift version:** `swift-driver version: 1.115.1 Apple Swift version 6.0.3 (swiftlang-6.0.3.1.10 clang-1600.0.30.1)` / my Package.swift file is set to `5.9`

### Checklist
- [ ] If possible, I've reproduced the issue using the `main` branch of this package
- [x] I've searched for [existing GitHub issues](https://github.com/apple/swift-algorithms/issues)

### Steps to Reproduce
Replace this paragraph with an explanation of how to reproduce the incorrect behavior. Include a simple code example, if possible.

### Expected behavior
Compilation to work

### Actual behavior
I get an error about importing `_NumericShims`.

(marked as a draft because I'm still doing a bit of troubleshooting, but I wanted to throw it out there in case I don't have time to get back to it)

Start by comparing the Swift Algorithms 1.2.1 and 1.2.0 package configurations, using the reported Swift 6.0.3 toolchain and Package.swift tools version 5.9. Reproduce the build and inspect the complete `_NumericShims` import error; done means version 1.2.1 builds successfully with the stated configuration.

Written by the indexing model from the issue text.