address review comments

tools/run_tests/artifacts/build_artifact_csharp.sh

@@ -20,8 +20,13 @@ cd "$(dirname "$0")/../../.."
 mkdir -p cmake/build
 cd cmake/build
 
-cmake -DCMAKE_BUILD_TYPE=RelWithDebInfo -DgRPC_BACKWARDS_COMPATIBILITY_MODE=ON -DgRPC_BUILD_TESTS=OFF "${CMAKE_ARCH_OPTION}" ../..
-make grpc_csharp_ext
+cmake -DCMAKE_BUILD_TYPE=RelWithDebInfo \
+      -DgRPC_BACKWARDS_COMPATIBILITY_MODE=ON \
+      -DgRPC_BUILD_TESTS=OFF \
+      "${CMAKE_ARCH_OPTION}" \
+      ../..
+
+make grpc_csharp_ext -j2
 cd ../..
 
 mkdir -p "${ARTIFACTS_OUT}"

tools/run_tests/helper_scripts/pre_build_csharp.sh

@@ -15,7 +15,7 @@
 
 set -ex
 
-# cd to gRPC csharp directory
+# cd to repository root
 cd "$(dirname "$0")/../../.."
 
 mkdir -p cmake/build

tools/run_tests/run_tests.py

@@ -1021,6 +1021,8 @@ class CSharpLanguage(object):
         if self.platform == 'windows':
             return 'cmake/build/%s/Makefile' % self._cmake_arch_option
         else:
+            # no need to set x86 specific flags as run_tests.py
+            # currently forbids x86 C# builds on both Linux and MacOS.
             return 'cmake/build/Makefile'
 
     def dockerfile_dir(self):