can't get Xcode not to build x86_64 for Swift Packages

I'm trying to improve my build time on macOS by not building for x86_64. I've got the following settings:

This gets Xcode not to build x86_64 for my app, but not all the package dependencies.

I've updated most of the packages to swift-tools-version: 6.0 but FlatBuffers is still on 5.8 and .macOS(.v10_14). GPT claims:

If your deployment target is set to macOS 10.15 or earlier, Xcode may force x86_64 support for compatibility reasons.

But Xcode is building x86_64 for ALL my packages, even the ones that don't depend on FlatBuffers.

When I open a package in Xcode that depends on FlatBuffers, then it builds arm only, so that may be a red herring.

Not sure what else to try.

can't get Xcode not to build x86_64 for Swift Packages
 
 
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