Specify host triple for mock Swift SDK in testUserToolchainCompileFlags

Author: MaxDesiatov

Tests/BuildTests/BuildPlanTests.swift

@@ -4356,6 +4356,7 @@ final class BuildPlanTests: XCTestCase {
         XCTAssertNoDiagnostics(observability.diagnostics)
 
         let userSwiftSDK = try SwiftSDK(
+            hostTriple: hostTriple,
             toolset: .init(
                 knownTools: [
                     .cCompiler: .init(extraCLIOptions: ["-I/fake/sdk/sysroot", "-clang-flag-from-json"]),