PackageModel: pass along -sdk property on Windows

If the user specifies `-sdk` always prefer that over the `SDKROOT`
environment variable.  We would previously honor that but then fail to
pass along the argument.  We never noticed this as `SDKROOT` has always
been set up until this point.  With newer SDKs, this will become less a
guarantee.

Author: compnerd

Sources/PackageModel/UserToolchain.swift

@@ -271,7 +271,7 @@ public final class UserToolchain: Toolchain {
             return destination.extraSwiftCFlags
         }
 
-        return (triple.isDarwin() || triple.isAndroid() || triple.isWASI()
+        return (triple.isDarwin() || triple.isAndroid() || triple.isWASI() || triple.isWindows()
                 ? ["-sdk", sdk.pathString]
                 : [])
         + destination.extraSwiftCFlags