sdk: fix permission issue when copying clang resources (#255)

## What does this PR do?

Fixes #254 by using `cp -R` instead of `FileManager.copyItem`. The
latter tries to preserve the original owner of files instead of using
the caller, which fails if you're non-root and the original files are
root-owned.

## How was it tested?

I was able to repro the original issue by installing an SDK as a
non-root user in an xtool Docker container.

Applied this patch, tried `xtool sdk install` again, and it worked.

## AI tool usage

How much of this PR was AI-assisted? (check one)

- [ ] **0** - No AI was used to write code
- [x] **1** - I was assisted by AI. I reviewed the finished result.
- [ ] **2** - I set the AI going and left it to it; nobody has read the
result - no review, or AI review only

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Sources/XToolSupport/SDKCommand.swift

@@ -171,7 +171,7 @@ struct DarwinSDK {
         let hostClangResources = URL(filePath: output.trimmingCharacters(in: .whitespacesAndNewlines))
         let hostInclude = hostClangResources.appending(path: "include")
         let sdkInclude = sdk.appending(path: "Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/swift/clang/include")
-        try FileManager.default.copyItem(at: hostInclude, to: sdkInclude)
+        try await FileManager.default.copyItem(at: hostInclude, to: sdkInclude, preserveOwner: false)
     }
 
     static func current() async throws -> DarwinSDK? {
@@ -264,7 +264,7 @@ struct InstallSDKOperation {
 
         if path.hasSuffix(".xtoolsdk") {
             print("Installing prebuilt SDK...")
-            try FileManager.default.copyItem(at: URL(filePath: path), to: sdkPath)
+            try await FileManager.default.copyItem(at: URL(filePath: path), to: sdkPath, preserveOwner: false)
         } else {
             // validate input before removing existing SDK
             let input = try SDKBuilder.Input(path: path)

Sources/XUtils/Foundation+Utils.swift

@@ -1,4 +1,7 @@
 import Foundation
+#if canImport(Subprocess)
+import Subprocess
+#endif
 
 extension Data {
     // AsyncBytes is Darwin-only :/
@@ -20,6 +23,30 @@ extension Data {
     }
 }
 
+extension FileManager {
+    /// Like `copyItem(at:to:)` but allows opting out of preserving the owner
+    package func copyItem(
+        at srcURL: URL,
+        to dstURL: URL,
+        preserveOwner: Bool,
+    ) async throws {
+        // why this is needed: https://github.com/xtool-org/xtool/issues/254
+
+        #if canImport(Subprocess)
+        if !preserveOwner {
+            try await Subprocess.run(
+                .name("cp"),
+                arguments: ["-R", srcURL.path, dstURL.path],
+                output: .discarded,
+            ).checkSuccess()
+            return
+        }
+        #endif
+
+        try self.copyItem(at: srcURL, to: dstURL)
+    }
+}
+
 package func stderrPrint(_ message: String, terminator: String = "\n") {
     try? FileHandle.standardError.write(contentsOf: Data("\(message)\(terminator)".utf8))
 }