Support cancellation of installation process (#770)

We wrap the installation with a withTaskCancellationHandler, which
ensures that the SIGINT signal handling code in main() will trigger
a cancellation of the installer.

As the VZMacOSInstaller must be both created and interacted with
on the VM's queue, which in our case is the main queue, we need
to move the logic to a separate function tagged with @MainActor.
This makes sense either way, as it cleans up the code a bit.

Sources/tart/VM.swift

@@ -191,18 +191,24 @@ class VM: NSObject, VZVirtualMachineDelegate, ObservableObject {
       virtualMachine.delegate = self
 
       // Run automated installation
-      try await withCheckedThrowingContinuation { (continuation: CheckedContinuation<Void, Error>) in
-        DispatchQueue.main.async { [ipswURL] in
-          let installer = VZMacOSInstaller(virtualMachine: self.virtualMachine, restoringFromImageAt: ipswURL)
+      try await install(ipswURL)
+    }
 
-          defaultLogger.appendNewLine("Installing OS...")
-          ProgressObserver(installer.progress).log(defaultLogger)
+    @MainActor
+    private func install(_ url: URL) async throws {
+      let installer = VZMacOSInstaller(virtualMachine: self.virtualMachine, restoringFromImageAt: url)
+      defaultLogger.appendNewLine("Installing OS...")
+      ProgressObserver(installer.progress).log(defaultLogger)
 
+      try await withTaskCancellationHandler(operation: {
+        try await withCheckedThrowingContinuation { continuation in
           installer.install { result in
             continuation.resume(with: result)
           }
         }
-      }
+      }, onCancel: {
+        installer.progress.cancel()
+      })
     }
   #endif