Move cross compile mixin into swiftpm base only

This was erroneously added to mixin_swiftpm_base (the base of SwiftPM +
*all* dependents) rather than SwiftPM's base only
(mixin_swiftpm_macos_platform). Move it down.

Author: bnbarham

utils/build-presets.ini

@@ -1846,14 +1846,15 @@ swift-testing-macros
 install-swift-testing
 install-swift-testing-macros
 
-infer-cross-compile-hosts-on-darwin
 skip-test-swift
 
 [preset: mixin_swiftpm_macos_platform]
 mixin-preset=
     mixin_swiftpm_base
     mixin_buildbot_install_components_with_clang
 
+infer-cross-compile-hosts-on-darwin
+
 [preset: mixin_swiftpm_linux_platform]
 mixin-preset=
     mixin_swiftpm_base