Swift plugins are incorrectly built using cross compilation sdk

[rauhul]

Description

Package plugins are incorrectly built using target sdk settings when using the --experimental-swift-sdk flag.

This results in the plugin failing to build in my case because my cc SDK doesn't contain a module required for the plugin (Foundation). Additionally my sdk has a toolset.json that uses a number of compiler and linker flags entirely incompatible with the host platform.

[rauhul]

cc @MaxDesiatov

[neonichu]

IIRC, we are already building plugins in a separate build invocation, with parameters created via this method. So I think we "just" need to tweak that to do the right thing.