XCTest is not fully open-source, only the Swift library parts. We don't have a
command-line runner available, so disable support.

--- a/Sources/Commands/Utilities/TestingSupport.swift
+++ b/Sources/Commands/Utilities/TestingSupport.swift
@@ -72,7 +72,7 @@ enum TestingSupport {
     /// - Returns: Array of TestSuite
     static func getTestSuites(fromTestAt path: AbsolutePath, swiftTool: SwiftTool, enableCodeCoverage: Bool, sanitizers: [Sanitizer]) throws -> [TestSuite] {
         // Run the correct tool.
-        #if os(macOS)
+        #if false
         let data: String = try withTemporaryFile { tempFile in
             let args = [try Self.xctestHelperPath(swiftTool: swiftTool).pathString, path.pathString, tempFile.path.pathString]
             var env = try Self.constructTestEnvironment(
--- a/Sources/swiftpm-xctest-helper/main.swift
+++ b/Sources/swiftpm-xctest-helper/main.swift
@@ -11,8 +11,11 @@
 //===----------------------------------------------------------------------===//
 
 #if os(macOS)
-import XCTest
 import func Darwin.C.exit
+print("Not supported in Nix.")
+exit(1)
+#if false
+import XCTest
 
 /// A helper tool to get list of tests from a XCTest Bundle on macOS.
 ///
@@ -134,6 +137,7 @@ do {
     exit(1)
 }
 
+#endif // nix
 #else
 
 #if os(Windows)