PInvokeTableGenerator: Find path to WasmAppBuilder.dll at test time

.. instead of depending on *one* runtime pack version. This is needed
when we have more than one workload installed.

src/mono/wasm/Wasm.Build.Tests/PInvokeTableGeneratorTests.cs

@@ -462,9 +462,21 @@ public static void Main()
 
             """;
 
+            string tasksDir = Path.Combine(s_buildEnv.WorkloadPacksDir,
+                                                              "Microsoft.NET.Runtime.WebAssembly.Sdk",
+                                                              s_buildEnv.GetRuntimePackVersion(DefaultTargetFramework),
+                                                              "tasks");
+            if (!Directory.Exists(tasksDir))
+                throw new DirectoryNotFoundException($"Could not find tasks directory {tasksDir}");
+
+            string? taskPath = Directory.EnumerateFiles(tasksDir, "WasmAppBuilder.dll", SearchOption.AllDirectories)
+                                            .FirstOrDefault();
+            if (string.IsNullOrEmpty(taskPath))
+                throw new FileNotFoundException($"Could not find WasmAppBuilder.dll in {tasksDir}");
+
             projectCode = projectCode
                 .Replace("###WasmPInvokeModule###", AddAssembly("System.Private.CoreLib") + AddAssembly("System.Runtime") + AddAssembly(libraryBuildArgs.ProjectName))
-                .Replace("###WasmAppBuilder###", Path.Combine(s_buildEnv.WorkloadPacksDir, "Microsoft.NET.Runtime.WebAssembly.Sdk", s_buildEnv.GetRuntimePackVersion(), "tasks", DefaultTargetFramework, "WasmAppBuilder.dll"));
+                .Replace("###WasmAppBuilder###", taskPath);
 
             buildArgs = buildArgs with { ProjectName = $"icall_enum_{buildArgs.Config}_{id}", ProjectFileContents = projectCode };