Merge pull request #21657 from jtattermusch/workaround_kokoro_golang_problem

workaround C# build failure by ensuring Go1.13 is installed

tools/internal_ci/helper_scripts/prepare_build_macos_rc

@@ -107,6 +107,20 @@ then
 
   # Xcode 10+ doesn't support building for i386 architecture
   sudo xcode-select -switch /Applications/Xcode_9.2.app/Contents/Developer/
+
+  # make sure we have the golang 1.13+
+  # recent golang is currently required by boringssl's cmake build
+  # and C# uses cmake to build grpc_csharp_ext
+  # see https://github.com/grpc/grpc/issues/21626
+  which go
+  ls -l $(which go)
+  go version  # check original golang version (to diagnose the issue)
+  brew unlink go  # stop using brew-provided version of go
+  time curl -O https://dl.google.com/go/go1.13.6.darwin-amd64.pkg
+  time sudo installer -pkg ./go1.13.6.darwin-amd64.pkg -target /
+  export PATH="/usr/local/go/bin:$PATH"
+  which go
+  go version
 fi
 
 # PHP tests currently require using an older version of PHPUnit