[CI] Added a gRPC_BUILD_TESTS guard to third_party protos (grpc#38179)

When not having submodules under the `third_party` directory, CMake throws an error in building gRPC because it couldn't find proto files in those directories even though those files are only used for tests.

```
CMake Error at CMakeLists.txt:611 (file):
  file COPY cannot find
  "/root/grpc/third_party/googleapis/google/api/annotations.proto": No such
  file or directory.
Call Stack (most recent call first):
  CMakeLists.txt:1184 (protobuf_generate_grpc_cpp_with_import_path_correction)
```

There's a interesting quirk in the CMake build process:

- Empty `third_party/googleapis` directory: Build fails.
- No `third_party/googleapis` directory: Build succeeds.

This inconsistency went undetected by our distrib-test because it completely removes the third_party/googleapis directory. However, Cloud C++ encountered this issue because it downloads a tarball that creates an empty third_party/googleapis directory.

This fix hides those proto targets when `gRPC_BUILD_TESTS` is not turned on to not run ino this problem.

Closes grpc#38179

COPYBARA_INTEGRATE_REVIEW=grpc#38179 from veblush:cmake-proto b8c9ca9
PiperOrigin-RevId: 699286689

templates/CMakeLists.txt.template

@@ -713,11 +713,25 @@
     DEPENDS tools_c tools_cxx)
 
   % for src in sorted(protobuf_gen_files):
+  % if not src.startswith('third_party/'):
   protobuf_generate_grpc_cpp_with_import_path_correction(
     ${src} ${third_party_proto_import_path(src)}
   )
+  % endif
   % endfor
 
+  # This enables CMake to build the project without requiring submodules
+  # in the third_party directory as long as test builds are disabled.
+  if (gRPC_BUILD_TESTS)
+  % for src in sorted(protobuf_gen_files):
+  % if src.startswith('third_party/'):
+  protobuf_generate_grpc_cpp_with_import_path_correction(
+    ${src} ${third_party_proto_import_path(src)}
+  )
+  % endif
+  % endfor
+  endif()
+
   if(gRPC_BUILD_TESTS)
     add_custom_target(buildtests_c)
     % for tgt in targets: