[swiftpm] Fix the bin path we pass to hostDestination

There is not guarantee that the toolchain.path contains `usr/bin`.
Instead, look at the diretory containing `swiftc`.

Author: benlangmuir

Sources/SKSwiftPMWorkspace/SwiftPMWorkspace.swift

@@ -69,7 +69,7 @@ public final class SwiftPMWorkspace {
 
     self.packageRoot = resolveSymlinks(packageRoot)
 
-    guard let destinationToolchainBinDir = toolchainRegistry.default?.path?.appending(components: "usr", "bin") else {
+    guard let destinationToolchainBinDir = toolchainRegistry.default?.swiftc?.parentDirectory else {
         throw Error.cannotDetermineHostToolchain
     }