[build-script] Don't build for iOS etc. if only SwiftPM is needed

Author: ahoppen

utils/build-presets.ini

@@ -1468,10 +1468,10 @@ skip-test-xctest
 [preset: mixin_swiftpm_package_macos_platform]
 mixin-preset=mixin_swiftpm_macos_platform
 
-# Build stdlib for all platforms.
-ios
-tvos
-watchos
+# We don't need these platforms. Skip them
+skip-ios
+skip-tvos
+skip-watchos
 
 skip-test-llbuild
 skip-test-swiftpm