Use x86 protoc binaries on arm64 Windows (#32017)

src/csharp/Grpc.Tools.Tests/ProtoToolsPlatformTaskTest.cs

@@ -79,6 +79,11 @@ namespace Grpc.Tools.Tests
                 {
                     Assert.AreEqual("x64", _task.Cpu);
                 }
+                // On windows arm64, x86 is used until a native protoc is shipped
+                else if (RuntimeInformation.IsOSPlatform(OSPlatform.Windows))
+                {
+                    Assert.AreEqual("x86", _task.Cpu);
+                }
                 else
                 {
                     Assert.AreEqual("arm64", _task.Cpu);

src/csharp/Grpc.Tools/ProtoToolsPlatform.cs

@@ -65,6 +65,11 @@ namespace Grpc.Tools
             {
                 Cpu = "x64";
             }
+            // Use x86 on Windows arm64 until a native protoc is shipped
+            else if (Os == "windows" && Cpu == "arm64")
+            {
+                Cpu = "x86";
+            }
 
             return true;
         }